Simplify mscw-reader path requirements, read year_available from files

[heikoklein]

Change Summary

• The mscw-reader no longer reads unknown model-files. Known model-files are now clearly stated instead of guessed by globs.
• There is no longer a requirement of 4-digits in the data-dir which were interpreted as year.
• years-available are now read from the model-files like Base_fullrun.nc
• Trends/multi-year analysis can be given as a data-dir to a directory containing subdirectories with four-digits. The subdirectories must contain Base_*.nc files (with * being hour, day, month, fullrun)
• the filepath attribute was never used and therefore removed

Related issue number

fixes #896

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 93.33333% with 5 lines in your changes missing coverage. Please review.

Project coverage is 79.32%. Comparing base (b1fa238) to head (ea8d117).
Report is 447 commits behind head on main-dev.

Files with missing lines	Patch %	Lines
pyaerocom/io/mscw_ctm/reader.py	93.33%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           main-dev    #1245      +/-   ##
============================================
- Coverage     79.33%   79.32%   -0.02%     
============================================
  Files           133      133              
  Lines         20289    20301      +12     
============================================
+ Hits          16096    16103       +7     
- Misses         4193     4198       +5     

Flag	Coverage Δ
unittests	79.32% <93.33%> (-0.02%)	⬇️

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[lewisblake]

Minor changes. Looks good otherwise