Fix xarray warning

[thorbjoernl]

Change Summary

Missed an xarray warning in #1502 which didn't surface in the tests surprisingly enough.

FutureWarning: In a future version of xarray decode_timedelta will default to False rather than None. To silence this warning, set decode_timedelta to True, False, or a 'CFTimedeltaCoder' instance.
  with xr.open_mfdataset(list(UEMEP_PATH.glob("*.nc")), engine="netcdf4") as dt:

This sets decode_timedelta to true for all opendataset calls, which maintains the old behaviour.

Related issue number

#1066

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 48.21429% with 58 lines in your changes missing coverage. Please review.

Project coverage is 78.34%. Comparing base (d90ad0f) to head (c27e575).
Report is 65 commits behind head on main-dev.

Files with missing lines	Patch %	Lines
pyaerocom/io/pyaro/postprocess.py	47.16%	56 Missing ⚠️
pyaerocom/colocation/colocation_utils.py	0.00%	1 Missing ⚠️
pyaerocom/io/cams2_83/reader.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           main-dev    #1505      +/-   ##
============================================
- Coverage     78.49%   78.34%   -0.16%     
============================================
  Files           140      140              
  Lines         21299    21405     +106     
============================================
+ Hits          16719    16769      +50     
- Misses         4580     4636      +56     

Flag	Coverage Δ
unittests	78.34% <48.21%> (-0.16%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.