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.

np.matrix is not only slow, but it is an ill-behaved sequence:
m[0].ndim == m.ndim == 2

This causes problems when converting a numpy matrix to a tinyarray.

Instead of a tuple of tuples of python integers, return a list of numpy arrays.
Initialize the arrays by memcpy.

There is no point in converting that purpose-made list into a tuple.