Custom variables when running aeroval

[dulte]

Adds possibility to add custom variables without hardcoding

Related issue number

Fix #1146

Checklist

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

[codecov]

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 ⚠️
pyaerocom/io/mscw_ctm/reader.py	73.33%	4 Missing ⚠️
pyaerocom/config.py	88.88%	2 Missing ⚠️
pyaerocom/aeroval/modelentry.py	85.71%	1 Missing ⚠️

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%)	⬇️

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[jgriesfeller]

Part of the code looks quite familiar from the Readpyaro extension #1124. Anyway, let's merge this first and I will adjust #1124 accordingly. But please remove the commented out method.
Please also correct the linting.

[jgriesfeller]

all good now