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)

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

Attention: Patch coverage is 23.80952% with 48 lines in your changes missing coverage. Please review.

Project coverage is 94.30%. Comparing base (d6eaac2) to head (b4f46b9).

Files	Patch %	Lines
esmvalcore/preprocessor/_derive/pfr.py	23.80%	48 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2358      +/-   ##
==========================================
- Coverage   94.62%   94.30%   -0.32%     
==========================================
  Files         246      247       +1     
  Lines       14055    14118      +63     
==========================================
+ Hits        13299    13314      +15     
- Misses        756      804      +48     

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