fix(get_transmissivities): support DISU

[wpbonelli]

Follow up on #2611 to support DISU. Closes #2170.

Plus forward-looking refactoring in case we add more grid types before 4.x comes out. Rather than conditionals everywhere to access/modify grid properties differently based on the grid type, add an adapter exposing what's needed consistently. This is necessary because grid properties may have different shapes depending on the grid type. In 4.x this can be avoided by design I think. But for now this makes the essence of the transmissivities calculation clearer.

[codecov]

Codecov Report

❌ Patch coverage is 87.09677% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.3%. Comparing base (556c088) to head (7158dc3).
⚠️ Report is 111 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/utils/postprocessing.py	87.0%	12 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2685      +/-   ##
===========================================
+ Coverage     55.5%    72.3%   +16.8%     
===========================================
  Files          644      667      +23     
  Lines       124135   130284    +6149     
===========================================
+ Hits         68947    94275   +25328     
+ Misses       55188    36009   -19179     

Files with missing lines	Coverage Δ
flopy/utils/postprocessing.py	80.0% <87.0%> (+2.4%)	⬆️

... and 562 files with indirect coverage changes

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