Fluxes get computed redundantly

Currently, we compute fluxes redundantly on both sides of the interface. To avoid that, we'd need an actual mortar discretization, i.e. something with one (surface) element per (facial) element interface. The closest we currently get is "two elements per interface". Of course, we would also need connections (likely in meshmode) to get onto and off of that mortar space.

cc @kaushikcfd @thomasgibson @lukeolson @MTCam @majosm