wence/feature/plex submesh

This change, which is mostly work by Koki Sagiyama (@ksagiyam) at Imperial (who I will tag in as soon as he has a gitlab account), allows extracting arbitrary dimension sub meshes from a DMPlex object.

The idea is that you build the submesh by providing labelled points at any height, and you get back a submesh that contains the closure of those points. The pointSF on the submesh is rewired to fix up a bunch of ownership cornercases (for example when extracting a submesh such that an interior shared face becomes an exterior boundary), tested in a new test (ex37.c).

I have not checked if this stuff could subsume the existing plexsubmesh functionality yet. Maybe it can.