Add snow age RGB recipe for VII/METimage

[adybbroe]

Add snow age RGB recipe for VII/METimage and use Oscar naming for channel specs in the reader yaml file

• Closes #xxxx
• Tests added
• Fully documented
• Add your name to AUTHORS.md if not there already

The METimage sensor does have compatible bands to VIIRS concerning the otherwise VIIRS unique snow_age RGB composite. Below an overview of the relevant spectral responses.

VIIRS only:

Both METimage and VIIRS:

And here results of two nearly coincident scenes from both sensors over Scandinavia.

First METimage around 08:40 UTC February 13, 2026:

NB! DISCLAIMER - METimage data now shown here are early commissioning data

And here VIIRS from around 08:49 from Suomi-NPP the same day:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.35%. Comparing base (6c571f2) to head (bd923ca).
⚠️ Report is 100 commits behind head on main.

Additional details and impacted files

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  Coverage   96.35%   96.35%           
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  Files         466      466           
  Lines       59083    59083           
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  Hits        56931    56931           
  Misses       2152     2152           

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[ameraner]

The new composite looks good to me.

However what is the advantage of using the numbered channel names in the yaml dataset keys, instead of the wavelength names as in the name? I see that the important bit is anyway the name, since it's what is used in composites and also in scn.available_dataset_names(). Maybe we could add the numbered channel name as an extra key in the dataset attrs and keep the key and the name consistent (using the wavelength)? Although they're used in OSCAR, I find the numbered names not very user friendly, and some numbers in between are even missing...