Grid validity check

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Hello, I want to help add a feature to automatically check the grid validity at the end and graphically flag any cells with problems. The shapely library makes it easy to check that cells are not twisted (polygon.is_valid) and that they touch but don't overlap their neighbors (polygon1.touches(polygon2)). I've added the is_valid check, but the overlap check is more subtle because the poloidal indices jump at target plates and X-points. Since I am not very familiar with the codebase yet, is there a simple way, compatible across the different topologies, to get the indices of the physically adjacent cells?

The issue names Shapely's polygon.is_valid and polygon.touches, plus poloidal indices, target plates, and X-points; begin by tracing how those topologies define physically adjacent cells. Compare the existing is_valid addition with the grid-generation flow, then define a topology-independent adjacency check and graphical problem marking. Done means twisted cells and neighboring overlaps are checked across each topology.

Written by the indexing model from the issue text.