chore: remove cvfdutil.gridlist_to_disv_gridprops function

[wpbonelli]

Close #2660. Needs an update in the nested grid MF6 example before CI will pass

Includes a bit of test rewriting, in particular where Lgr properly handles hanging vertices.

Also add a Lgr.from_parent_grid() classmethod for convenience. It's a bit verbose to manually pass all the parent grid props into the Lgr grid initializer, this makes it easier moved to #2668

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.6%. Comparing base (556c088) to head (229c307).
⚠️ Report is 92 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2667      +/-   ##
===========================================
+ Coverage     55.5%    72.6%   +17.1%     
===========================================
  Files          644      667      +23     
  Lines       124135   129382    +5247     
===========================================
+ Hits         68947    94015   +25068     
+ Misses       55188    35367   -19821     

Files with missing lines	Coverage Δ
flopy/utils/cvfdutil.py	83.7% <ø> (+1.1%)	⬆️
flopy/utils/lgrutil.py	97.2% <100.0%> (+<0.1%)	⬆️

... and 558 files with indirect coverage changes

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