explicitely cast to datetime64[ns] (non-ns-compat)

[mathause]

• Closes upstream dev issues hidden by pinned xarray #601
• Tests added
• Fully documented, including CHANGELOG.rst

pandas and xarray relax the condition that datetime has to be in nano second resolution. Explicitly cast to datetime64[ns] so our tests don't fail. (Could be better to use seconds instead but for backwards compatibility we can make our live easier.) Mostly done so I can remove it from #607.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.40%. Comparing base (32a5eb0) to head (b6647f2).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #608   +/-   ##
=======================================
  Coverage   80.40%   80.40%           
=======================================
  Files          49       49           
  Lines        3082     3082           
=======================================
  Hits         2478     2478           
  Misses        604      604           

Flag	Coverage Δ
unittests	80.40% <100.00%> (ø)

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