Create MeshIndexSet class

[trexfeathers]

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

Should be a 'view' onto an existing Mesh, but with all Coords and Connectivitys subsetted appropriately. See #6120 for more detail on the philosophy.

See #6014 and satellite pull requests for ideas so far.

• Create the class
• Devise a way to cope with indexing a MeshIndexSet - to instead index the original Mesh
• Write tests for creation and operations
• Don't worry about using the class in Iris operations yet - this is separated in Use MeshIndexSet class for subsetting MeshCoord #6122.