improve/fix xr splitter

[pawel-wolff]

xr_engine splitter improved/fixed in case "split_dims" contains multiple keys

If to_xarray converter gets "split_dims" keyword, a slight change of behaviour is introduced: it returns a tuple with:

• a list of xarray Datasets
• a corresponding list of dictionaries with values of the spitting keys.
  Moreover, the two above are always lists, even if they are of length 1 (for consistency of the behaviour).

TBD if we really want to introduce this new behaviour since the values of the splitting keys can be obtained via using "ensure_dims" keyword of to_xarray.

A test and a corresponding mid-size test dataset was added.

[FussyDuck]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.01%. Comparing base (5dd3946) to head (20b6171).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #688   +/-   ##
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  Coverage    91.01%   91.01%           
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  Files          162      162           
  Lines        12385    12391    +6     
  Branches       609      609           
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+ Hits         11272    11278    +6     
  Misses         932      932           
  Partials       181      181           

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[sandorkertesz]

@pawel-wolff, thank you very much for this fix and improvement in the Xarray engine. I fully agree with your decision to change the return type when split_dims=True.