Index MeshCoord using new MeshXY or _MeshIndexSet

[trexfeathers]

🚀 Pull Request

Description

Closes #6122

To do

• Second pass of the code
• Tests
• Adjust task list of Implement the UGRID Location Index Set #6120 to reflect the interim option of indexing using a new Mesh

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[trexfeathers]

Some notes after revisiting:

• I went hard to avoid the parent class (MeshXY) needing to know about the subclass (_MeshIndexSet), since that would obviously be an anti-pattern. But in doing so I just added a load of other anti-patterns instead (e.g. self.__class__ = ...). The better way to achieve this would be a _MeshXYMixin¹, providing common behaviour for both MeshXY and _MeshIndexSet, but easily allowing for completely different __init__ etcetera.
• I need to find a way to have the docs render _MeshIndexSet, while still keeping it private:
  https://github.com/trexfeathers/iris/blob/41cbe93b2d6c4a69fc9a0b8ed759540f5f9c5a42/lib/iris/mesh/components.py#L2021-L2023

Footnotes

1. _MeshMixin in future: https://github.com/SciTools/iris/blob/57a316c8a7ebfb4052081537f408204ab0781ba5/lib/iris/mesh/components.py#L598-L608 ↩