add tutorial name (and number) to tutorial examples

doc/source/tutorial/ab_ring.py

+# Tutorial 2.3.3. Nontrivial shapes
+# =================================
+#
 # Physics background
 # ------------------
 #  Flux-dependent transmission through a quantum ring

doc/source/tutorial/band_structure.py

+# Tutorial 2.4.1. Band structure calculations
+# ===========================================
+#
 # Physics background
 # ------------------
 #  band structure of a simple quantum wire in tight-binding approximation

doc/source/tutorial/closed_system.py

+# Tutorial 2.4.2. Closed systems
+# ==============================
+#
 # Physics background
 # ------------------
 #  Fock-darwin spectrum of a quantum dot (energy spectrum in

doc/source/tutorial/graphene.py

+# Tutorial 2.5. Beyond square lattices: graphene
+# ==============================================
+#
 # Physics background
 # ------------------
 #  Transport through a graphene quantum dot with a pn-junction

doc/source/tutorial/plot_graphene.py

+# Tutorial 2.7.1. 2D example: graphene quantum dot
+# ================================================
+#
 # Physics background
 # ------------------
 #  - graphene edge states

doc/source/tutorial/plot_zincblende.py

+# Tutorial 2.7.2. 3D example: zincblende structure
+# ================================================
+#
+# Physical background
+# -------------------
+#  - 3D Bravais lattices
+#
+# Kwant features highlighted
+# --------------------------
+#  - demonstrate different ways of plotting in 3D
+
 import kwant
 from matplotlib import pyplot
 

doc/source/tutorial/quantum_well.py

+# Tutorial 2.3.2. Spatially dependent values through functions
+# ============================================================
+#
 # Physics background
 # ------------------
 #  transmission through a quantum well

doc/source/tutorial/quantum_wire.py

+# Tutorial 2.2.2. Transport through a quantum wire
+# ================================================
+#
 # Physics background
 # ------------------
 #  Conductance of a quantum wire; subbands

doc/source/tutorial/quantum_wire_revisited.py

+# Tutorial 2.2.3. Building the same system with less code
+# =======================================================
+#
 # Physics background
 # ------------------
 #  Conductance of a quantum wire; subbands

doc/source/tutorial/spin_orbit.py

+# Tutorial 2.3.1. Matrix structure of on-site and hopping elements
+# ================================================================
+#
 # Physics background
 # ------------------
 #  Gaps in quantum wires with spin-orbit coupling and Zeeman splititng,

doc/source/tutorial/superconductor_band_structure.py

+# Tutorial 2.6.1. Superconductors - "Orbital description": using matrices
+# =======================================================================
+#
 # Physics background
 # ------------------
 #  band structure of a superconducting quantum wire in tight-binding

doc/source/tutorial/superconductor_transport.py

+# Tutorial 2.6.2. "Lattice description": using different lattices
+# ===============================================================
+#
 # Physics background
 # ------------------
 # - conductance of a NS-junction (Andreev reflection, superconducting gap)