mars2grib: Support ocean fields

[tweska]

This PR adds support for ORCA grids in the representation concept and adds level mappings required to encode ocean fields.

This PR is meant to be used with these changes in MultIO: ecmwf/multio#247

[codecov-commenter]

Codecov Report

❌ Patch coverage is 1.85185% with 53 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.33%. Comparing base (87efe4a) to head (2223a27).
⚠️ Report is 7 commits behind head on develop.

Files with missing lines	Patch %	Lines
...it/mars2grib/backend/concepts/level/levelMatcher.h	0.00%	29 Missing ⚠️
...d/concepts/representation/representationEncoding.h	6.66%	14 Missing ⚠️
...etkit/mars2grib/backend/concepts/level/levelEnum.h	0.00%	9 Missing ⚠️
...nd/concepts/representation/representationMatcher.h	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #181      +/-   ##
===========================================
- Coverage    62.61%   62.33%   -0.28%     
===========================================
  Files          303      303              
  Lines        11554    11601      +47     
  Branches      1028     1036       +8     
===========================================
- Hits          7234     7232       -2     
- Misses        4320     4369      +49     

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[dsarmany]

Thank you.