Add custom CMOR tables for ESACCI-PERMAFROST

[axel-lauer]

Description

This PR adds custom CMOR tables for the following three ESACCI-PERMAFROST variables:

• active layer thickness (alt)
• permafrost ground temperature (gtd)
• permafrost extent (pfr)

and a derivation script for permafrost extent (pfr) to calculate this variable from the CMIP models.

Checklist

It is the responsibility of the author to make sure the pull request is ready to review. The icons indicate whether the item will be subject to the 🛠 Technical or 🧪 Scientific review.

• 🧪 The new functionality is relevant and scientifically sound
• 🛠 This pull request has a descriptive title and labels
• 🛠 Code is written according to the code quality guidelines
• 🧪 and 🛠 Documentation is available
• 🛠 Unit tests have been added
• 🛠 Changes are backward compatible
• 🛠 Any changed dependencies have been added or removed correctly
• 🛠 The list of authors is up to date
• 🛠 All checks below this pull request were successful

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 45 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.31%. Comparing base (bf60005) to head (f4fb071).

Files with missing lines	Patch %	Lines
esmvalcore/preprocessor/_derive/pfr.py	25.00%	45 Missing ⚠️

❌ Your patch check has failed because the patch coverage (25.00%) is below the target coverage (100.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2358      +/-   ##
==========================================
- Coverage   95.59%   95.31%   -0.28%     
==========================================
  Files         266      267       +1     
  Lines       15559    15619      +60     
==========================================
+ Hits        14873    14888      +15     
- Misses        686      731      +45     

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[github-actions]

In order to maintain a backlog of relevant pull requests, we automatically label them as stale after 180 days of inactivity.

If this pull request is still important to you, please comment below to remove the stale label. Otherwise, this pull request will be automatically closed in 60 days. If this pull request only suffers from a lack of reviewers, please tag the @ESMValGroup/technical-lead-development-team so they can help you find a suitable reviewer.

[bouweandela]

It looks like there is also a derivation function in this pull request that is not mentioned in the description at the top. Is that intentional? If yes, please make sure that the derivation is lazy and add a unit test for it.