Before we used the include_dir argument to setup(), but this caused problems:
kwant/kwant#48

Commit a8b0463e turned out to be a bad
idea: unlike on Debian, on many platforms the numpy headers are not
available in the default include path.

    1. Now can be compiled without MUMPS
       (previously exception was thrown)
    2. Gives sensible error message for Git install,
       but without installed Cython.

With the reorganization of setup.py, most constants are now used in a
single place.  It is clearer and simpler to have them there.

The CYTHON_TRACE macro can be now set in the build.conf file.

All C extensions are now treated on equal footing and can be configured
in "build.conf".  All the options known by Extension may be set now for
any extension individually and for all of them together (using the
[DEFAULT] section).

This reduces the number of global variables and makes the code easier to
understand.

The removed code predates "build.conf" and should not be necessary on
most (all?) platforms.  Should numpy be missing in the include path, it
can be added using "build.conf".

If this change turns out to cause problems, we can always revert it.

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