Traversal tests, bug fix

[wx4stg]

Ran into this bug when running traversal with ecb_tobac_timeseries.py. Newer xarray groupby operations give single element slices instead of integer indices. This forces integer indices

Also brought over tests from glmtools.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 75.16%. Comparing base (97f8aaa) to head (31a3f75).

Files with missing lines	Patch %	Lines
pyxlma/lmalib/traversal.py	50.00%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
+ Coverage   73.20%   75.16%   +1.96%     
==========================================
  Files          16       16              
  Lines        2086     2090       +4     
==========================================
+ Hits         1527     1571      +44     
+ Misses        559      519      -40     

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[deeplycloudy]

Could you see if this has any impact on deeplycloudy/glmtools#104?

[wx4stg]

The check_parent_child method is not implemented on the pyxlma version of traversal, so this bug isn't present. Is there any interest in adding that method here?