Added interior facet + mixed/vector fs test
Merge pull request #232 from firedrakeproject/int-facet-mfs-test

Add tests of zero form interior facet integrals on both vector and mixed
function spaces.
DMPlex: Optimise plex_renumbering
DMPlex: Optimising the derivation of cell DoFs
DMPlex: Optimising mark_entity_classes
DMPlex: Optimise facet DoF derivation
DMPlex: Cleaning up PETSc interfaces

Move PETSc definitions into separate include file and add error handling.