Support automatic addition of supplementary variables for CORDEX

[bouweandela]

Description

Idea to improve a bit on #3031

Recipe for testing this with CORDEX data:

documentation:
  title: |
    Example recipe that loads CORDEX-CMIP5 datasets.
  description: |
    This is an example recipe that loads CORDEX-CMIP5 datasets.
    It computes the climatology for surface temperature (ts).
  authors:
    - andela_bouwe
  maintainer:
    - andela_bouwe

preprocessors:
  ts_pp:
    custom_order: true
    climate_statistics:
      period: full
      operator: mean
    mask_landsea:
      mask_out: sea

datasets:
  - dataset: CCLM4-8-17
    driver: ICHEC-EC-EARTH
    ensemble: r12i1p1
    institute: CLMcom
    rcm_version: v1

diagnostics:
  bias:
    variables:
      ts:
        project: CORDEX
        mip: day
        domain: EUR-11
        exp: historical
        timerange: 2003/2005
        preprocessor: ts_pp
    scripts:
      plot:
        plot_folder: '{plot_dir}'
        plot_filename: '{plot_type}_{real_name}_{alias}_{mip}'
        facet_used_for_labels: 'alias'
        script: monitor/multi_datasets.py
        plots:
          map:
            show_stats: false

To check that it works correctly, ensure that the sea is masked in the resulting plot.

Link to documentation: https://docs.esmvaltool.org/projects/ESMValCore/en/latest/recipe/overview.html#automatic-definition-of-supplementary-variables

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.16%. Comparing base (96095e3) to head (8c83f90).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3098   +/-   ##
=======================================
  Coverage   96.16%   96.16%           
=======================================
  Files         270      270           
  Lines       15837    15837           
=======================================
  Hits        15230    15230           
  Misses        607      607           

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