xarray utils fix, tests

[wx4stg]

Fixes #58 and adds tests for xarray_util module

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.31%. Comparing base (97f8aaa) to head (7541af5).
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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #71      +/-   ##
==========================================
+ Coverage   73.20%   75.31%   +2.10%     
==========================================
  Files          16       16              
  Lines        2086     2086              
==========================================
+ Hits         1527     1571      +44     
+ Misses        559      515      -44     

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