2010 generic unstructured dataset

[fluidnumericsJoe]

• Chose the correct base branch (main for v3 changes, v4-dev for v4 changes)
• Fixes Create generic unstructured dataset, resembling FESOM2, from delaunay triangulation and uniform vertical grid spacing #2010
• Added tests
• Added documentation

This PR adds a new unstructured grid generic dataset resembling FESOM. A test has been included in tests/v4, but it is marked to be skipped since there are some correctness issues in the _search_time_indices function that cause it to fail.

[VeckoTheGecko]

Looks good! Happy to merge

Just a note that it may be useful at some point in future to add a time dimension to these datasets (similar to what is done in structured.generic) where we defined T=13 and then do a xr.date_range("2000", "2001", T).

[fluidnumericsJoe]

These do have a time dimension ("time"). the thinking here is to provide a dataarray that would be consistent with what we get from loading a FESOM dataset in UxArray.

At any rate, I see your point. Could you open an issue describing the change you want to see ? That way we can get it knocked out in some of the "finishing/polishing" work and not forget this.