Implement the UGRID Location Index Set

[trexfeathers]

✨ Feature Request

A new class that references an existing Mesh, offering access to all its properties/operations/etcetera, but actually subsetting that Mesh's Coords and Connectivitys as described by the UGRID Location Index Set - effectively a 'view' on the original Mesh.

This work is to be performed on the FEATURE_index_set feature branch.

Motivation

This would provide a fuller implementation of UGRID within Iris, and is a viable solution to the following existing issues:

• 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.

Additional context

• Original proposal: Investigate: subsetting a MeshCoord should return a MeshCoord? #5991 (comment)
• First code attempts: Proof of concept: MeshIndexSet #6014

### Tasks
- [ ] https://github.com/SciTools/iris/issues/6121
- [ ] https://github.com/SciTools/iris/issues/6122
- [ ] https://github.com/SciTools/iris/issues/5939
- [ ] https://github.com/SciTools/iris/issues/6123
- [ ] Investigate: `MeshIndexSet` use in `Mesh` decomposition and recombination