Custom variables when running aeroval

[dulte]

Adds possibility to add custom variables without hardcoding

Related issue number

Fix #1146

Checklist

• [x] Start with a draft-PR
• [x] The PR title is a good summary of the changes
• [x] PR is set to AeroTools and a tentative milestone
• [ ] Documentation reflects the changes where applicable
• [x] Tests for the changes exist where applicable
• [x] Tests pass locally
• [ ] Tests pass on CI
• [x] At least 1 reviewer is selected
• [x] Make PR ready to review

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 78.84615% with 11 lines in your changes missing coverage. Please review.

Project coverage is 78.82%. Comparing base (63877b7) to head (0c8316f). Report is 582 commits behind head on main-dev.

Files	Patch %	Lines
pyaerocom/io/mscw_ctm/additional_variables.py	20.00%	4 Missing :warning:
pyaerocom/io/mscw_ctm/reader.py	73.33%	4 Missing :warning:
pyaerocom/config.py	88.88%	2 Missing :warning:
pyaerocom/aeroval/modelentry.py	85.71%	1 Missing :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##           main-dev    #1147      +/-   ##
============================================
- Coverage     79.35%   78.82%   -0.54%     
============================================
  Files           128      128              
  Lines         20162    20209      +47     
============================================
- Hits          16000    15930      -70     
- Misses         4162     4279     +117     

Flag	Coverage Δ
unittests	78.82% <78.84%> (-0.54%)	:arrow_down:

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