Optimize computation of timeseries statistics through multiprocessing

[lewisblake]

Change Summary

Timeseries were computed independently and in serial by looping over the regions. This PR parallelizes this for loop using multiprocessing. To that end, implements a function in pyaerocom/aeroval/coldatatojson_helpers.py, _process_statistics_timeseries_single_region, to compute the timeseries for a specific region, and uses multiprocessing in pyaerocom/aeroval/coldatatojson_engine.py to call this function. I also corrected some type hints.

Related issue number

Partly addresses #1277

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 53.84615% with 6 lines in your changes missing coverage. Please review.

Project coverage is 78.44%. Comparing base (9591a7d) to head (1fab191).
Report is 17 commits behind head on main-dev.

Files with missing lines	Patch %	Lines
pyaerocom/aeroval/coldatatojson_helpers.py	14.28%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           main-dev    #1492      +/-   ##
============================================
- Coverage     78.46%   78.44%   -0.02%     
============================================
  Files           139      139              
  Lines         21119    21130      +11     
============================================
+ Hits          16570    16576       +6     
- Misses         4549     4554       +5     

Flag	Coverage Δ
unittests	78.44% <53.84%> (-0.02%)	⬇️

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[lewisblake]

Note afabd57 contains the changes to parallelize the computations of the heat maps and scatter plots which were deemed outside of the scope of this PR in Pyaerocom Meeting 2025-01-27. This is because it requires parallel writing of files, which is a thorny problem we can't currently handle in aerovaldb. I'm keeping note of the commit here in case we want to revisit this problem.