Mat zeros

Fill newly created matrices with zeros, so petsc doesn't compact the space.

Additionally, rework sparsity construction to remove code duplication and implement parallel sparsity construction for all extruded cases.

AFAICT, this doesn't change the speed of sparsity building.
I've removed those.  the pyop2 requirements.txt has numpy/petsc4py in it.  I'm just copying the firedrake setup.py (in terms of how to build against petsc/numpy).