update Mf6Splitter: Add support for groundwater energy model

[jlarsen-usgs]

Closes #2415

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.3%. Comparing base (bb9824e) to head (54d9114).
Report is 35 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2418     +/-   ##
=========================================
+ Coverage     68.4%   76.3%   +7.8%     
=========================================
  Files          294     294             
  Lines        59390   59826    +436     
=========================================
+ Hits         40652   45666   +5014     
+ Misses       18738   14160   -4578     

Files with missing lines	Coverage Δ
flopy/mf6/utils/model_splitter.py	71.9% <100.0%> (+13.1%)	⬆️

... and 247 files with indirect coverage changes