1339 add pm ratios to EMEP reader; add example config for them

[jgriesfeller]

Change Summary

This PR adds two more calculated variables to the EMEP reader:
ratpm10pm25 and ratpm25pm10.
In addition, the aeroval config has been added which is available through
pyaerocom.aeroval.config.pmratios.base_config.get_CFG()

Related issue number

fix #1339

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 94.87179% with 2 lines in your changes missing coverage. Please review.

Project coverage is 78.53%. Comparing base (28a7cab) to head (61682b8).
Report is 19 commits behind head on main-dev.

Files with missing lines	Patch %	Lines
pyaerocom/io/mscw_ctm/additional_variables.py	90.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           main-dev    #1378      +/-   ##
============================================
+ Coverage     78.50%   78.53%   +0.03%     
============================================
  Files           136      137       +1     
  Lines         20860    20899      +39     
============================================
+ Hits          16377    16414      +37     
- Misses         4483     4485       +2     

Flag	Coverage Δ
unittests	78.53% <94.87%> (+0.03%)	⬆️

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[thorbjoernl]

LGTM