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

Attention: Patch coverage is 0% with 186 lines in your changes missing coverage. Please review.

Project coverage is 93.47%. Comparing base (11a4787) to head (01ef2d4).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
esmvalcore/cmor/_fixes/oras5/oras5.py	0.00%	130 Missing ⚠️
esmvalcore/cmor/_fixes/oras5/_base_fixes.py	0.00%	56 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2422      +/-   ##
==========================================
- Coverage   94.68%   93.47%   -1.21%     
==========================================
  Files         251      253       +2     
  Lines       14372    14558     +186     
==========================================
  Hits        13608    13608              
- Misses        764      950     +186     

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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.