Later these will become part of the official System interface

Modify tutorial to remove warnings.

Closes #304.

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.

Closes #263.

Also add reference documentation for 'density'.

Previously, in order to not to fix the matplotlib backend, we required
users to import matplotlib.pyplot before calling any Kwant plotting
functions.  This did not have the desired effect, since we did import
`matplotlib.backends` and that also fixes the backend.

Now, both backends and pyplot are imported at the last possible moment
and a warning is emitted if this fixes the backend.

Previously the fact that the positive direction of mode
velocity is determined by the translational symmetry vector
of the system was not made clear in the relevant docstrings.

Closes #143.

Add the 'unicode-math' package and set the
latex engine to 'xelatex'.

Closes #126.

Previously a point defect introduced by an incorrect treatment
of the origin produced erroneous scattering. We also update the text
to reflect the actual magnetic potential used.

Closes #156.

I assume that most people encounter the tutorial example scripts by
reading the documentation, and not by viewing the tutorial subdirectory
of a checked-out Kwant source.

That's the motivation for moving all the manipulations of tutorial
scripts from setup.py to doc/Makefile.  Previously, a successful 'make
html' would require a preceding execution of 'setup.py build_tut'.  Now,
a simple 'make html' is enough.

While at it, I reorganized where the example scripts and their outputs
are stored.  Everything is now in 'doc/source/code' with its three
subdirectories 'download' (to be shown to readers), 'include' (with
include markers), and 'figure' (figure generation & figures).  This
organization is clearer and also makes the generation of figures
separate from the tutorial.