DMPlex: mark non-exec-halo facets

Interior facets that only touch halo vertices /and/ halo cells need to
be marked as non-exec: we don't have enough data to compute over them,
but the data needs to be there such that assembly can execute
redundantly in parallel.

This actually produces exec halos that are slightly too large for fully
discontinuous fields: in that case we don't care about halo vertices,
only halo cells.  However, rather than specialising the code dependent
on the function space we just take the (hopefully minor) hit.
DMPlex: add non-exec halo facets to plex_reordering and facet_ordering
DMPlex: add non-exec halo to dof_classes
Enable interior facets in parallel

Now that we mark non-exec interior facets correctly, interior facet
integrals should work, so remove NotImplementedError from appropriate
assembly and expected failure from parallel upwind flux test.

dS_h integrals (extruded interior horizontal facets) are still broken,
so leave that error in.
Add __name__ == '__main__' magic to test_upwind_flux
Add DG advection test (parallel too)
Run 2D and 3D steady advection tests in parallel

Now that we can do interior facet integrals in parallel, test steady
advection in parallel as well as serial.
DMPlex: plug small memory leak
DMPlex: only restore closure at very end of closure_ordering