Feature/update xarray earthkit accessor

[sandorkertesz]

Description

This PR adds the following properties/methods to the Xarray earthkit accessor:

• reference_field: returns the reference field for a dataarray.
• set(*args, **kwargs): allows changing the reference field in the dataarray using the same args/kwargs as in Field.set()

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 19.51220% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.32%. Comparing base (c715146) to head (24970a3).
⚠️ Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
tests/xr_engine/test_xr_engine_accessor.py	20.00%	32 Missing ⚠️
tests/xr_engine/test_xr_engine_attrs.py	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #990      +/-   ##
===========================================
- Coverage    81.49%   81.32%   -0.17%     
===========================================
  Files          235      236       +1     
  Lines        15275    15308      +33     
  Branches       751      751              
===========================================
+ Hits         12448    12449       +1     
- Misses        2602     2634      +32     
  Partials       225      225              

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