Use `MeshIndexSet` class for subsetting `MeshCoord`

[trexfeathers]

Needs #6121 first

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

closes #4751, closes #4671

• Use MeshIndexSet in MeshCoord.__getitem__()
• Make sure this is functional when indexing a Cube with MeshCoords attached
• Make this experimental opt-in behaviour (need to iron out the quirks of MeshIndexSet)
• Avoid breaking changes by implementing this suggestion
• Adapt any existing uses of Mesh.from_coords() that are in Iris code and documentation - many/all might no longer be necessary
• Write tests