DMPlex: Marking all points in the Plex according to PyOP2 entity classes.
DMPlex: Reorder sections to create PyOP2-compatible DM/SF.

Reordering the default global and local section created from the
plex is required to sort DoFs into contiguous PyOP2 entity classes:
core, owned, exec-halo. The DefaultSF defining the halo-exchange
can then be derived from these, allowing the DM to perform the halo
exchange.