Custom user-defined location for custom CMOR tables

[schlunma]

Description

This PR allows the specification of a custom location for the custom CMOR tables in the config-developer.yml file (using the custom key).

Closes #1623

Link to documentation:

• https://esmvaltool--1625.org.readthedocs.build/projects/ESMValCore/en/1625/quickstart/find_data.html#datasets-in-native-format
• https://esmvaltool--1625.org.readthedocs.build/projects/ESMValCore/en/1625/quickstart/configure.html#custom-cmor-tables
• https://esmvaltool--1625.org.readthedocs.build/projects/ESMValCore/en/1625/develop/fixing_data.html#add-support-for-new-native-datasets

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[codecov]

Codecov Report

Merging #1625 (c9943b3) into main (1d94671) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1625      +/-   ##
==========================================
+ Coverage   91.38%   91.40%   +0.01%     
==========================================
  Files         204      204              
  Lines       11173    11188      +15     
==========================================
+ Hits        10211    10226      +15     
  Misses        962      962              

Impacted Files	Coverage Δ
esmvalcore/cmor/table.py	94.29% <100.00%> (+0.17%)	⬆️

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[sloosvel]

How is this going? Looks quite useful, but unfortunately now that all tests are passing, I have to continue with the release procedure. So, apologies but this won't be included at the end.

[schlunma]

The code is finished, just need to add some bits of doc and a tiny test. Would love to hear the opinion of @zklaus before I go on, though!

Don't worry, this doesn't need to go into rc1. If we need an rc2 (which is more than likely given past experiences), we can still include this.

[schlunma]

Since there are no objections I will proceed with this PR now.

[sloosvel]

Tested for the following cases:

• Not setting the custom directory in config-developer
• Setting the custom directory as a relative path
• Setting the custom directory in a custom location

Works in all three cases as expected. Further comments and reviews are welcome, but I will merge on Monday if no one opposes.