fix(model_splitter.py): origin can be taken from base model for unstructured grids

[mjr-deltares]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.5%. Comparing base (7eb7c88) to head (ff5624e).
Report is 24 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2514      +/-   ##
===========================================
- Coverage     76.1%    55.5%   -20.7%     
===========================================
  Files          296      644     +348     
  Lines        64215   124157   +59942     
===========================================
+ Hits         48885    68922   +20037     
- Misses       15330    55235   +39905     

Files with missing lines	Coverage Δ
flopy/mf6/utils/model_splitter.py	74.9% <100.0%> (-0.2%)	⬇️

... and 484 files with indirect coverage changes

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[mjr-deltares]

Hi @jlarsen-usgs , thanks for looking at this. The point is that the origins can be taken from the base model in case of DISV and DISU. Let me know if anything is unclear.

[jlarsen-usgs]

@mjr-deltares the changes look good to me