Fix Healpix encoding

[stefaniereuter]

Description

Healpix case in RepresentationMatcher was missing, and in the representationEncoding Nside key is with Capital letter and orderingConvention expects a string. If you prefer we can use ordering, and keep the previous logic, which would work with the numerical value.

Contributor Declaration

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• 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 20.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.02%. Comparing base (0e5d0f6) to head (2047694).

Files with missing lines	Patch %	Lines
...d/concepts/representation/representationEncoding.h	0.00%	3 Missing ⚠️
...nd/concepts/representation/representationMatcher.h	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #199      +/-   ##
===========================================
- Coverage    62.03%   62.02%   -0.01%     
===========================================
  Files          303      303              
  Lines        11671    11672       +1     
  Branches      1049     1049              
===========================================
  Hits          7240     7240              
- Misses        4431     4432       +1     

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[tweska]

Looks good to me. I ran clang-format to make the CI happy.

[pgeier]

climate dts HEALPix output get encoded correctly with this change