fix(optimize_splitting_mask): add edge weights for LAK cells

[jlarsen-usgs]

• refactor hfb edge weighting code
• add testing

[codecov]

Codecov Report

❌ Patch coverage is 97.67442% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 73.7%. Comparing base (556c088) to head (a0ba869).
⚠️ Report is 234 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/mf6/utils/model_splitter.py	97.6%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2853      +/-   ##
===========================================
+ Coverage     55.5%    73.7%   +18.2%     
===========================================
  Files          644      659      +15     
  Lines       124135   132450    +8315     
===========================================
+ Hits         68947    97729   +28782     
+ Misses       55188    34721   -20467     

Files with missing lines	Coverage Δ
flopy/mf6/utils/model_splitter.py	72.8% <97.6%> (-2.1%)	⬇️

... and 585 files with indirect coverage changes

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[jlarsen-usgs]

Closes #2827