Test that parallel spheres have no exterior facets
Fix parallel exterior facet labelling

We must label the exterior facets (the boundary) of the mesh before
distribution, since otherwise domain boundaries will be incorrectly
marked as exterior.
Merge pull request #414 from firedrakeproject/parallel_ext_facet

* parallel_ext_facet:
  Fix parallel exterior facet labelling
  Test that parallel spheres have no exterior facets