add combine nested

[malmans2]

Closes #274

I think the datasets that are failing are experiencing a breaking change in xarray. We just have to specify combine="nested".

@ThomasHaine looks like you tested with a different environment. I couldn't reproduce the issue using the Oceanography Image on SciServer (I believe that's because that image uses an old version of xarray).
Could you please run your script again with this version of the catalog?
You just have to specify the catalog url corresponding to this branch:

catalog_url="https://raw.githubusercontent.com/hainegroup/oceanspy/add-combine/sciserver_catalogs/catalog_xarray.yaml"
od = ospy.open_oceandataset.from_catalog(name, catalog_url=catalog_url)

(where name is the name of the catalogue entry, such as IGPwinter)

[codecov]

Codecov Report

Merging #279 (d359517) into main (a39fb85) will not change coverage.
The diff coverage is n/a.

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##             main     #279   +/-   ##
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  Coverage   95.11%   95.11%           
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  Lines        3848     3848           
  Branches      812      812           
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  Hits         3660     3660           
  Misses        122      122           
  Partials       66       66           

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[ThomasHaine]

Works for IGPwinter and EGshelfIIseas2km_ERAI_1D as expected.