Support for unstructured regional ML gridded data

[James Warner (jwarner8)]

Addresses #1975 and enables us to use CSET with regional ML models (BRIS, Regional FastNet).

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• Documentation has been updated to reflect change.
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[github-actions]

[James Warner (jwarner8)]

Have this working in the workflow, but aggregation very slow... there is a question of whether there is a better way, as we are regridding the data again in every baked recipe... over a 100 times.

[James Warner (jwarner8)]

Need to check how nan is propagated, given stats values showing np.nan