Fix writing of VTU files in parallel

Fixes #149
Merge pull request #218 from firedrakeproject/fix-parallel-vtu-write

Fix for parallel VTU output.

Fixes #149.
Add support for extruded facet integrals
Tidy existing tests, and un-xfail
Laplace and Poisson tests for facets, using Neumann BCs
Add interior facet stress tests (P0 and P1DG steady advection)