fix(model_splitter.py): add trap for obs packages in bc packages

[jlarsen-usgs]

• Fix indexing error in observation remapping routine
• Add test for remapping package observations

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.1%. Comparing base (e2af433) to head (4daa909).
Report is 10 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #2493   +/-   ##
=======================================
  Coverage     76.0%   76.1%           
=======================================
  Files          296     296           
  Lines        64158   64167    +9     
=======================================
+ Hits         48803   48848   +45     
+ Misses       15355   15319   -36     

Files with missing lines	Coverage Δ
flopy/mf6/utils/model_splitter.py	75.0% <100.0%> (+0.9%)	⬆️

... and 8 files with indirect coverage changes

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