Fix path in demos Makefile
Fix gitignore for new path structure
Adapt to changes in PyOP2 matrix assembly

The Mat user must now call assemble explicitly to ensure the matrix is
built before using it.
Allow exterior facet integrals in parallel
Set up fieldsplit ISes correctly for parallel running

Numbering of fields on process N is offset by the sum of the rows of the
big matrix on processes 0, ..., N-1 (inclusive).
Test mixed poisson problem in parallel with fieldsplit pc