Merge branch 'jrwrigh/isoperiodic_plex_mat_preallocate' into 'main'

fix(Plex): Fix Mat preallocation when isoperiodic is used

See merge request petsc/petsc!8712

plex: Correct Mat preallocation for isoperiodic grids

Requires:
- Using isoperiodic SF instead of the (purely topological) pointSF
- Force the use of the isoperiodic SF even if in serial
- Handle th

plex: Correct Mat preallocation for isoperiodic grids

Requires:
- Using isoperiodic SF instead of the (purely topological) pointSF
- Force the use of the isoperiodic SF even if in serial
- Handle the case of an isoperiodic SF leaf-root pair a) being on the
same rank and b) both being present in the same adjancency

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