feat(get_transmissivities): interpret head=None as fully saturated

[wpbonelli]

Close #2610

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.3%. Comparing base (556c088) to head (6f41142).
⚠️ Report is 116 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2690      +/-   ##
===========================================
+ Coverage     55.5%    72.3%   +16.8%     
===========================================
  Files          644      667      +23     
  Lines       124135   130394    +6259     
===========================================
+ Hits         68947    94377   +25430     
+ Misses       55188    36017   -19171     

Files with missing lines	Coverage Δ
flopy/utils/postprocessing.py	80.1% <100.0%> (+2.5%)	⬆️

... and 562 files with indirect coverage changes

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[wpbonelli]

the model should not default to None, it is required

[wpbonelli]

heads should also not default to None, just interpret an explicitly passed None as fully saturated