We use Cython's import mechanism to use the the same lapack used
by Scipy, so we no longer need to explicitly link against lapack
or blas.

Our oldest supported distribution is Ubuntu 16.04, which ships
with Python 3.5.

Closes #169

Generalize 'plotter.current_interpolation' to work for 1D and 3D,
and slightly refactor the code with more thorough comments.

The smoothing function is pure, and we should make this manifestly
clear.

In principle we could write it in a *completely* general way, but this
would involve numerically calculating the surface elements and
integrating the smoothing function. Given that the only useful cases are
1D, 2D, and 3D it makes sense to just hard-code the prefactors for these
cases and be done with it.

"grid" parameter takes role of "grid_spacing" and extends its
functionality. It can take scalar values, as "grid_spacing",
as well as "kwant.lattice.Monatomic" object. Currently there
are strong contraints of accepted lattice but they can be lowered
later when new functionality will be provided. "grid_spacing" is
being deprecated.

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.