Plex: Deriving point ownership of the exported Plex in parallel

This is part one of deriving the SF required to build a valid
parallel Plex object.
Plex: Establish remote roots for all points in the halo/non-core region

This done by first matching local vertices to their remote Plex IDs
via Pragmatic's GNN, before matching local entities by communicating
cones and matching the remote roots to local leafs with DMPlexGetJoin().
Plex: Build the Plex SF mapping local leafs to remote roots

This now allows us to export a valid parallel DMPlex object.