Fix/xr engine dims

[pawel-wolff]

Description

This PR:

• Fixes several bugs in the Xarray engine related to dimension discovery, squeezing vs. ensuring dimensions, and transforming dimensions into variable attributes.
• Provides more consistent behaviour for Xarray engine options related to dimensions and attributes handling - such as extra_dims, ensure_dims, dims_as_attrs - by clearly distinguishing between dimension names and metadata keys, in line with the logic introduced by the dim_roles concept.
• Introduces the explicit name "<level_per_type>" for the template dimension implemented in the LevelPerTypeDim class. Under this name, the dimension can be referenced in the options such as ensure_dims and dims_as_attrs.
• Fixes the "mars" profile by removing several duplicates between attrs and variable_attrs, and moves "param" from attrs to variable_attrs.
• Makes the ensure_dims and dims_as_attrs options no longer exclusive: one can prevent a size-1 dimension from being squeezed by referencing it in ensure_dims, while at the same time promoting a coordinate of that dimension as a variable attribute.

All the above improvements and (slight) changes to Xarray engine behaviour are:

• Documented in the to_xarray() method docstring.
• Documented on the page "User guide - Xarray engine - Dimensions".
• Demonstrated with new notebooks.
• Covered by new tests.

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

❌ Patch coverage is 91.08911% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.81%. Comparing base (a0eca61) to head (dd3a684).

Files with missing lines	Patch %	Lines
tests/xr_engine/test_xr_dims.py	88.00%	3 Missing and 6 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #867      +/-   ##
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+ Coverage    85.78%   85.81%   +0.03%     
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  Files          184      184              
  Lines        14369    14468      +99     
  Branches       713      727      +14     
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+ Hits         12326    12416      +90     
- Misses        1838     1841       +3     
- Partials       205      211       +6     

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