Partial collapse of multi-dim string coords: take 2

[rcomer]

🚀 Pull Request

Description

This is the same bugfix as #4294. In there, I also tried to add type hints following my interpretation of the dev guide. This turned out to be complicated and I suspect is the main reason the PR has not made progress.

I think it would be good to fix the bug regardless of typing questions, so here is a version without the type hints which I'm hoping will be an easier review.

Fixes #3653.

If you have a 2D string coordinate on a cube, and collapse one of the dimensions spanned by that coordinate, the coordinate's points (and bounds, if any) are now serialised along the collapsed dimension. This seems to have been missed when partial collapse was implemented for the numeric coordinates.

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