Adding fix for ORAS5

[jmalles]

Description

Fix for ORAS5. It includes the option to make the 'irregular' (2d) grid 'unstructured' (1d) and UGRID-compliant. To do so, one needs to add the extra facet 'make_unstructured: true' and/or 'ugrid: true' in the recipe.

The mesh description ('horizontal_grid' in the recipe) needs to be read from a file (oras5_mesh.zip), which is based on a file that can be downloaded here. Since ORAS5 data is on the ORCA025 (Arakawa-C grid), there are different mesh files for tracers (T), and zonal/meridional velocities (u/v).

An example recipe for ORAS5 can be found here.

Some already downloaded data are stored here:
/work/bd1083/b380216/extraobsraw/Tier2/ORAS5/single_level
/work/bd1083/b380216/extraobsraw/Tier2/ORAS5/all_levels
/work/bd1083/b382555/extraobsraw/Tier2/ORAS5/all_levels

The grid files are also stored on Levante:
/work/bd0854/b382555/extraobsraw/Tier2/ORAS5/grids

Link to documentation: tba

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[codecov]

Codecov Report

❌ Patch coverage is 93.25843% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.53%. Comparing base (f7d004d) to head (9f61cb4).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
esmvalcore/cmor/_fixes/native6/oras5.py	93.25%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2422      +/-   ##
==========================================
- Coverage   95.55%   95.53%   -0.03%     
==========================================
  Files         263      264       +1     
  Lines       15508    15686     +178     
==========================================
+ Hits        14819    14985     +166     
- Misses        689      701      +12     

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[bettina-gier]

Tried with the example recipe and it looks good, output seems as expected. Fast look through the code looks good. I'll do a more detailed review when I have more time.

[schlunma]

Hi @jmalles, thanks for contributing!

To start the review process and finalize this, we need tests (see e.g., here) and some documentation (see e.g, here). Thanks!!