Semantics of xr-engine decode_timedelta kwarg fixed

[pawel-wolff]

Description

The decode_timedelta kwargs will conform to the docstring in the xr-engine method .to_xarray():
If the coordinate step has int or str values (e.g. when issued from the GRIB key step) and decode_timedelta=True, then a conversion to timedelta is applied. In other cases the step coordinate remain unchanged (either as int or timedelta, if it was issued from the step_timedelta key)

Several xr-engine tests were adjusted or corrected to accomodate this change.
Also, the coordinate comparison test (in xr-engine tests) was fixed.

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 85.35%. Comparing base (34b3656) to head (1397c9c).
⚠️ Report is 6 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #901      +/-   ##
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- Coverage    85.36%   85.35%   -0.01%     
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  Files          184      184              
  Lines        14573    14564       -9     
  Branches       732      723       -9     
===========================================
- Hits         12440    12431       -9     
  Misses        1922     1922              
  Partials       211      211              

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