Cache validation ignore pyaerocom version; use mtime instead of ctime

[heikoklein]

Change Summary

The cache validation of ungridded data was using the pyaercom_version to identify invalid caches. Since caches and pyaerocom_version are independent, this is now only logged, but no longer used to invalidate caches.

In addition, cache validation was using ctime to identify newest files, but ctime is metadata change time, i.e. when permissions/ownerships of data changed last. This has been changed to mtime which is data modification time.

Related issue number

fixes #1242

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 70.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 79.33%. Comparing base (f244a51) to head (b1fa238).
Report is 449 commits behind head on main-dev.

Files with missing lines	Patch %	Lines
pyaerocom/io/cachehandler_ungridded.py	70.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           main-dev    #1243   +/-   ##
=========================================
  Coverage     79.33%   79.33%           
=========================================
  Files           133      133           
  Lines         20284    20289    +5     
=========================================
+ Hits          16092    16096    +4     
- Misses         4192     4193    +1     

Flag	Coverage Δ
unittests	79.33% <70.00%> (+<0.01%)	⬆️

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[jgriesfeller]

Looks good