MDA8 timestamp and superfluous values

[thorbjoernl]

Change Summary

• MDA8 values will now have the interval midpoint as their timestamp (ie. 1pm / 13:00 instead of 1am / 01:00).
• MDA8 will not include calculated values for days which did not have at least one data point in the input timeseries.

Related issue number

• fix MDA8 calculation for single year results in ts of two years #1262
• fix MDA8 daily values have wrong timestamp #1263

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 (Except test_readgridded/test_years_available)
• Tests pass on CI
• At least 1 reviewer is selected
• Make PR ready to review

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.35%. Comparing base (816b2cc) to head (bb81351).
Report is 44 commits behind head on main-dev.

Additional details and impacted files

@@            Coverage Diff            @@
##           main-dev    #1265   +/-   ##
=========================================
  Coverage     79.34%   79.35%           
=========================================
  Files           133      133           
  Lines         20421    20424    +3     
=========================================
+ Hits          16204    16207    +3     
  Misses         4217     4217           

Flag	Coverage Δ
unittests	79.35% <100.00%> (+<0.01%)	⬆️

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[heikoklein]

Please try my suggestion, otherwise this looks fine and I approve.