Review all legacy issues which should be fixed or unblocked by new UGRID Location Index Set

[trexfeathers]

Test, close, update, contact stakeholders, as appropriate:

• Constrained load on MeshCoord bypasses callback #5939
• Implement Cube.remesh #4751
• MeshCoord cannot compare with an 'equivalent' ordinary AuxCoord #4671
• It has been suggested here that this would provide more natural 'stitching' of a decomposed mesh, currently only available via recombine_submeshes - a highly specific proof-of-concept.

(Remember that any work is to be performed on the FEATURE_index_set feature branch.)