add matplotlib boilerplate to start of all tutorials

Currently we just set the default figure size. Matplotlib is
annoying when it comes to setting these default parameters, and we
have to ensure that we set any 'rcParams' in a separate Jupyter cell
and *after* we import pyplot.

doc/source/pre/whatsnew/1.3.rst

@@ -52,6 +52,9 @@ a quantum anomalous spin Hall phase:
     import kwant
     import kwant.continuum
 
+.. jupyter-execute:: ../../tutorial/boilerplate.py
+    :hide-code:
+
 .. jupyter-execute::
 
     def make_model(a):

doc/source/tutorial/boilerplate.py

+import matplotlib
+import matplotlib.pyplot
+from IPython.display import set_matplotlib_formats
+
+matplotlib.rcParams['figure.figsize'] = matplotlib.pyplot.figaspect(1) * 2
+set_matplotlib_formats('svg')

doc/source/tutorial/discretize.rst

@@ -42,6 +42,9 @@ continuum models and for discretizing them into tight-binding models.
 
     import kwant
 
+.. jupyter-execute:: boilerplate.py
+    :hide-code:
+
 .. _tutorial_discretizer_introduction:
 
 Discretizing by hand

doc/source/tutorial/faq.rst

@@ -14,6 +14,9 @@ into Kwant's structure.
     import matplotlib
     from matplotlib import pyplot as plt
 
+.. jupyter-execute:: boilerplate.py
+    :hide-code:
+
 What is a system, and what is a builder?
 ========================================
 A Kwant system represents a particular tight-binding model. It contains a graph

doc/source/tutorial/first_steps.rst

@@ -88,6 +88,9 @@ In order to use Kwant, we need to import it:
     # For plotting
     from matplotlib import pyplot
 
+.. jupyter-execute:: boilerplate.py
+    :hide-code:
+
 .. jupyter-execute::
 
     import kwant
@@ -452,6 +455,11 @@ file and defining the a square lattice and empty scattering region.
     # For plotting
     from matplotlib import pyplot
 
+.. jupyter-execute:: boilerplate.py
+    :hide-code:
+
+.. jupyter-execute::
+
     a = 1
     t = 1.0
     W, L = 10, 30
@@ -661,6 +669,11 @@ return a Kwant ``Builder``:
     from matplotlib import pyplot
     import kwant
 
+.. jupyter-execute:: boilerplate.py
+    :hide-code:
+
+.. jupyter-execute::
+
     def make_system(L, W, a=1, t=1.0):
         lat = kwant.lattice.square(a)
 

doc/source/tutorial/graphene.rst

@@ -47,6 +47,9 @@ explicitly here to show how to define a new lattice:
 
     sin_30, cos_30 = (1 / 2, sqrt(3) / 2)
 
+.. jupyter-execute:: boilerplate.py
+    :hide-code:
+
 .. jupyter-execute::
 
     graphene = kwant.lattice.general([(1, 0), (sin_30, cos_30)],

doc/source/tutorial/kpm.rst

@@ -39,6 +39,9 @@ KPM method `kwant.kpm`, that is based on the algorithms presented in Ref. [1]_.
     import scipy
     from matplotlib import pyplot
 
+.. jupyter-execute:: boilerplate.py
+    :hide-code:
+
 Introduction
 ************
 
@@ -151,7 +154,7 @@ We start by importing kwant and defining our system.
 
     # Plot several density of states curves on the same axes.
     def plot_dos(labels_to_data):
-        pyplot.figure(figsize=(5,4))
+        pyplot.figure()
         for label, (x, y) in labels_to_data:
             pyplot.plot(x, y.real, label=label, linewidth=2)
         pyplot.legend(loc=2, framealpha=0.5)
@@ -162,7 +165,7 @@ We start by importing kwant and defining our system.
 
     # Plot fill density of states plus curves on the same axes.
     def plot_dos_and_curves(dos, labels_to_data):
-        pyplot.figure(figsize=(5,4))
+        pyplot.figure()
         pyplot.fill_between(dos[0], dos[1], label="DoS [a.u.]",
                          alpha=0.5, color='gray')
         for label, (x, y) in labels_to_data:

doc/source/tutorial/operators.rst

@@ -51,6 +51,9 @@ texture.
     # letters denote spinor indices
     sigma = np.rollaxis(np.array([sigma_x, sigma_y, sigma_z]), 1)
 
+.. jupyter-execute:: boilerplate.py
+    :hide-code:
+
 Introduction
 ------------
 Our starting point will be the following spinful tight-binding model on

doc/source/tutorial/plotting.rst

@@ -35,6 +35,9 @@ these options can be used to achieve various very different objectives.
 
     import kwant
 
+.. jupyter-execute:: boilerplate.py
+    :hide-code:
+
 We begin by first considering a circular graphene quantum dot (similar to what
 has been used in parts of the tutorial :ref:`tutorial-graphene`.)  In contrast
 to previous examples, we will also use hoppings beyond next-nearest neighbors:
@@ -216,6 +219,9 @@ visible. The hoppings are also plotted in order to show the underlying lattice.
 
     import kwant
 
+.. jupyter-execute:: boilerplate.py
+    :hide-code:
+
 Zincblende is a very common crystal structure of semiconductors. It is a
 face-centered cubic crystal with two inequivalent atoms in the unit cell
 (i.e. two different types of atoms, unlike diamond which has the same crystal

doc/source/tutorial/spectrum.rst

@@ -30,6 +30,9 @@ Band structure calculations
     # For plotting
     from matplotlib import pyplot
 
+.. jupyter-execute:: boilerplate.py
+    :hide-code:
+
 When doing transport simulations, one also often needs to know the band
 structure of the leads, i.e. the energies of the propagating plane waves in the
 leads as a function of momentum. This band structure contains information about
@@ -137,6 +140,9 @@ Closed systems
     from matplotlib import pyplot
     import kwant
 
+.. jupyter-execute:: boilerplate.py
+    :hide-code:
+
 Although Kwant is (currently) mainly aimed towards transport problems, it
 can also easily be used to compute properties of closed systems -- after
 all, a closed system is nothing more than a scattering region without leads!

doc/source/tutorial/spin_potential_shape.rst

@@ -68,6 +68,9 @@ for small arrays.)
     # For plotting
     from matplotlib import pyplot
 
+.. jupyter-execute:: boilerplate.py
+    :hide-code:
+
 .. jupyter-execute::
 
     # For matrix support
@@ -242,6 +245,9 @@ Spatially dependent values through functions
     # For plotting
     from matplotlib import pyplot
 
+.. jupyter-execute:: boilerplate.py
+    :hide-code:
+
 Up to now, all examples had position-independent matrix-elements
 (and thus translational invariance along the wire, which
 was the origin of the conductance steps). Now, we consider the
@@ -403,6 +409,9 @@ Nontrivial shapes
     # For plotting
     from matplotlib import pyplot
 
+.. jupyter-execute:: boilerplate.py
+    :hide-code:
+
 Up to now, we only dealt with simple wire geometries. Now we turn to the case
 of a more complex geometry, namely transport through a quantum ring
 that is pierced by a magnetic flux :math:`\Phi`:
@@ -594,6 +603,12 @@ period of one flux quantum.
         import kwant
         from matplotlib import pyplot
 
+    .. jupyter-execute:: boilerplate.py
+        :hide-code:
+
+    .. jupyter-execute::
+        :hide-code:
+
         a = 1
         t = 1.0
         W = 10
@@ -643,6 +658,12 @@ period of one flux quantum.
         import kwant
         from matplotlib import pyplot
 
+    .. jupyter-execute:: boilerplate.py
+        :hide-code:
+
+    .. jupyter-execute::
+        :hide-code:
+
         a = 1
         t = 1.0
         W = 10

doc/source/tutorial/superconductors.rst

@@ -35,6 +35,9 @@ Superconductors: orbital degrees of freedom, conservation laws and symmetries
     tau_y = tinyarray.array([[0, -1j], [1j, 0]])
     tau_z = tinyarray.array([[1, 0], [0, -1]])
 
+.. jupyter-execute:: boilerplate.py
+    :hide-code:
+
 This example deals with superconductivity on the level of the
 Bogoliubov-de Gennes (BdG) equation. In this framework, the Hamiltonian
 is given as