Support loading ICON grid from ICON rootpath

[schlunma]

Description

Currently, the ICON grid file can either be specified via the facet horizontal_grid or automatically downloaded from a public website specified by the GRID_FILE_URI attribute of the data. Since the grid file is almost always present in the ICON output directory, this PR adds support for that. If possible, the ICON grid file is now retrieved from the ICON directory specified in the user configuration file.

Link to documentation: https://esmvaltool--2337.org.readthedocs.build/projects/ESMValCore/en/2337/quickstart/find_data.html#icon

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.13%. Comparing base (5941300) to head (9cd1798).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2337      +/-   ##
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+ Coverage   94.12%   94.13%   +0.01%     
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  Files         241      241              
  Lines       13387    13410      +23     
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+ Hits        12600    12623      +23     
  Misses        787      787              

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[diegokam]

The ICON grid file is loaded correctly from the ICON rootpath specified in the configuration file and the plots are generated properly.
It is not needed anymore to declare the absolute path of the grid file in the recipe(s).

[valeriupredoi]

looking good, just a few minor points from me, Manu 🍻

[valeriupredoi]

perfect in me 📖 s Manu 🍺