WIP: Add test if quantities work with matplotlib

This adds a test if quantities work with matplotlib (see issue #1881).

On my Mac this leads to `python setup.py test -P units` simply aborting without giving an error message.
So I presume this is not the right way to add this test?

cc @mdboom @astrofrog
@embray has been asking to always give test functions and classes a docstring -- might as well move these links there.