As reported by paraview/paraview#21347, point values were not shared
at the interface. It isn't in general a problem as points are supposed
to have the same values. It becomes a problem when one wants to keep
track of who is the owner of a ghost point at the interface by sharing a
process id array, for instance.

Given a point belonging to multiple partitions in the input, the
partition of lowest id in the DIY environment is set to be the owner of
this point. Blocks don't send interfaces to the blocks of lower id, but
receives them from the latter.

Making the ghost cells generator function in this instance required
those few changes:

* There is no more a shallow-copy when requesting zero layers of ghosts
on the PointData, as the point data array of the input is written over.
The CellData and the geometry are still shallow-copied.
* Computing the point ids to share needed reworked. Now, the owner of
the points at the interface sends them as we...