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.

Original Plan

• 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.

Update 2026-06-09

New pull request - #7149 - provides a draft implementation of #6121 and #6122 together, but without tests or full documentation. This is a much more natural, creative breakdown made possible by targeting a feature branch. Tests and docs should follow in subsequent issues (pending) and pull requests.

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[trexfeathers]

Still working on this in the background