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 25.00000% with 45 lines in your changes missing coverage. Please review.

Project coverage is 95.15%. Comparing base (bee9f04) to head (4db4a42).
Report is 3 commits behind head on main.

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

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2358      +/-   ##
==========================================
- Coverage   95.42%   95.15%   -0.28%     
==========================================
  Files         260      261       +1     
  Lines       15426    15486      +60     
==========================================
+ Hits        14720    14735      +15     
- Misses        706      751      +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.