Closes #166.

There is no point in repeating the docstring in 2 places, though we
leave in the explanation of what this means in the "simplest case", i.e.
lowest modes first, for clarity.

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.

The basic idea remains the same, but should work much better now.  The
difference is that images/example.py.diff is now a patch with complete
context and becomes the authoritative source for both the visible flavor
of an example as well as for its figure-generating variant.  Both
tutorial/example.py and images/example.py are extracted from this file
by 'make html'.  Thanks to the complete context the diffs are quite
readable and may be modified directly.

Alternatively, one may also modify the generated scripts.  When
tutorial/example.py has been modified, it will be patched and saved as
images/example.py.  The patching is done using the tool 'wiggle' that
works much better than 'patch'.  If a conflict occurs, conflict markers
are added to the output file and its dated back to the dawn of time
(i.e. 1970) in order to mark the conflicts as not yet resolved.  After
resolving, 'make html' is simply run again.

Upon a successful execution of the figure-generating script the diff
gets recreated automatically, 'generate-diffs' no longer exists.

These classes used not to have any features beyond
kwant.system.{Finite,Infinite}System, but this is no longer the case.

update the mode ordering documentation as it applies to kwant>=1.3.

Add an FAQ section to the Kwant tutorial that succinctly answers
user questions that have appeared on the mailing list a number of
times.

lowercase FAQ.rst, FAQ.py, etc., give meaningful names to labels,
and stop building an user-visible tutorial/faq.py

Apply pep8 fixes and refactor some sections to be more idiomatic
(e.g. avoiding triple nested loops). Also some minor reorganisation
of the sections to give the text a better flow.

The alleged mode ordering was the opposite of the true ordering.
The true ordering corresponds to "lowest modes first" when there are
no band crossing.

That test fails because the result of the selfenergy callback is used as a
numpy array without a call to asarray.

Calling "expand" at this point is not required and differs from
sympy behaviour without a good argument for it. Closes #146.

Older pytest does not have 'warns'.

Closes #132

 The total density of states now integrates to `N`, the size of the system.

This option is not necessary since a similar effect can be achieved by
filling an emtpy builder and using Builder.update.  Furthermore,
removing it opens the door for making fill "sticky" in a consistent way.