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.
Yes, I agree. OpenCL needs a major overhaul anyway once someone is vaguely interest in e.g. running on the Phi.