Set decode_timedelta=True to silence 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 71.42857% with 2 lines in your changes missing coverage. Please review.

Project coverage is 78.49%. Comparing base (d90ad0f) to head (07e65cb).
Report is 70 commits behind head on main-dev.

Files with missing lines	Patch %	Lines
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    #1506   +/-   ##
=========================================
  Coverage     78.49%   78.49%           
=========================================
  Files           140      140           
  Lines         21299    21299           
=========================================
  Hits          16719    16719           
  Misses         4580     4580           

Flag	Coverage Δ
unittests	78.49% <71.42%> (ø)

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