Parallel DG

This branch enables, finally, parallel DG integrals.  Effectively all the halo data was already there, we just needed to correctly construct the extents of the exec-halo and non-exec-halo for the interior facet iteration set.  With that done, the one remaining barrier is to actually let assembly of interior facets go through.

Additionally, enable the upwind flux reconstruction test in parallel, add a new dg advection test (parallel too) and modify the steady advection tests to run in parallel as well as serial.