Feature/xr engine aux coords

[pawel-wolff]

Description

This PR adds the aux_coords kwarg to the Xarray engine. It allows to declare 1- or multi-dimensional auxiliary coordinate variables whose values are derived from any metadata key (format-agnostic, format-specific or virtual, issued from remapping). The value of aux_coords, if not None, should be a dictionary with:

• key: an auxiliary coordinate label
• value: (metadata_key: str, dataset_dimension(s): str or iterable of str)

Dedicated tests added.

A new how-to notebook added.

As an extra:

• ListDiff._compare improved and now provides a sensible error when data do not form a hypercube

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.38%. Comparing base (5eb79ea) to head (5c7cf24).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #969      +/-   ##
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+ Coverage    81.26%   81.38%   +0.12%     
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  Files          223      224       +1     
  Lines        14624    14719      +95     
  Branches       718      718              
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+ Hits         11884    11979      +95     
  Misses        2521     2521              
  Partials       219      219              

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