I've realized that this was broken. (The tests were not installed and could be
only run for inplace-builds linked from somewhere else.)  Fixing this would be
somewhat hacky.  I think now that it was a bad idea in the first place to
advertise the running of unit tests to end users.

This new nomenclature avoids the confusion between "lead neighbors" and "site
neighbors".

The code should be easier to understand.  Configuration through a site.cfg file
is supported.

Relative cimports are not supported by Cython so far.  Once this has been
solved we can switch to relative cimports as well and kwant will be importable
under different names.