Extract interface for gridded modeldata reader

[heikoklein]

Change Summary

• extract interface for gridded-model-readers (addresses parts of Split pyaerocom into seperate projects #772 )
• apply interace to all know gridded-data readers
• reduce public api of mscw-ctm data reader to contain only the interface

Related issue number

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 96.20253% with 3 lines in your changes missing coverage. Please review.

Project coverage is 79.04%. Comparing base (9365dc0) to head (01a158a).
Report is 6 commits behind head on main-dev.

Files	Patch %	Lines
pyaerocom/io/cams2_83/reader.py	72.72%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           main-dev    #1269   +/-   ##
=========================================
  Coverage     79.04%   79.04%           
=========================================
  Files           135      136    +1     
  Lines         20766    20786   +20     
=========================================
+ Hits          16414    16431   +17     
- Misses         4352     4355    +3     

Flag	Coverage Δ
unittests	79.04% <96.20%> (+<0.01%)	⬆️

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[dulte]

Looks good

[lewisblake]

Small clean up of dead code but feel free to merge after removing.