This is part of the Scalable Matrix Extensions (SME)

otherwise we can delete stuff from the next outer loop, which in 2D would be the one OpenMP-parallelized

Fixes #25

Block.subs method tried to be too smart:
a = field[..]
b = a + b

was "simplified" incorrectly to
b = field[...] + b

This restructuring allows for easier separation of modules into
separate repositories later. Also, now pip install with repo url can be
used.

The setup.py files have also been updated to correctly reference each
other. Module versions are not extracted from git state