Fix(test_save_load_node_mapping_structured):

[jlarsen-usgs]

• update _node_map to _node_map_arr to follow new convention

• remove sklearn requirement as test was getting skipped and now sklearn is not required

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.5%. Comparing base (556c088) to head (9b24dfe).
⚠️ Report is 223 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2838      +/-   ##
===========================================
+ Coverage     55.5%    73.5%   +18.0%     
===========================================
  Files          644      659      +15     
  Lines       124135   132222    +8087     
===========================================
+ Hits         68947    97301   +28354     
+ Misses       55188    34921   -20267     

see 586 files with indirect coverage changes

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