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.

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.

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.

In addition to basic updates to skip permanent redirect, remove an
obsolete reference to macports installation instructions on the main
website.

Summary of changes:
* Add a test for trailing whitespace, remove all trailling whitespace
* Add a test for broken URLs in docs, fix all broken URLs and permanent redirects
* Switch sphinx doc build to strict (warnings to errors) and nitpicky (broken links to warnings). Fix all sphinx warnings.
* Deploy docs from any commit to [kwant/kwant](https://gitlab.kwant-project.org/kwant/kwant) to https://test.kwant-project.org/doc/dev
* Deploy docs from any commit to master at kwant/kwant to https://kwant-project.org/doc/dev
* Deploy docs from any tag release tag (so anything that is vX.Y.Z) at kwant/kwant to https://kwant-project.org/doc/X.Y

this fixes #35

It was not previously made explicit anywhere in the docs on how
the leads were numbered. This commit adds a line to the docstring
of `kwant.builder.Builder.attach_lead` which explains the meaning
of the return value. It also adds a few lines to the first tutorial
explaining the numbering.