preprocessing fixes and upates

[larsbuntemeyer]

some fixes for the preprocessing module

[codecov-commenter]

Codecov Report

Merging #36 (f26f6c2) into master (a2e1ff1) will decrease coverage by 5.93%.
The diff coverage is 49.48%.

@@            Coverage Diff             @@
##           master      #36      +/-   ##
==========================================
- Coverage   87.12%   81.18%   -5.94%     
==========================================
  Files          17       18       +1     
  Lines         598      675      +77     
==========================================
+ Hits          521      548      +27     
- Misses         77      127      +50     

Flag	Coverage Δ
unittests	81.18% <49.48%> (-5.94%)	⬇️

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Impacted Files	Coverage Δ
cordex/preprocessing/__init__.py	100.00% <ø> (ø)
cordex/preprocessing/known_issues.py	11.11% <11.11%> (ø)
cordex/core/domain.py	81.56% <47.82%> (-12.28%)	⬇️
cordex/preprocessing/preprocessing.py	80.09% <56.41%> (-6.50%)	⬇️
cordex/core/cf.py	100.00% <100.00%> (ø)