fix(lgrutil): remove duplicate hanging vertices

[wpbonelli]

Lgr.to_disv_gridprops() left duplicate hanging vertices in the generated grid. Fix it by adding deduplication logic after vertices are collected. Some variables to record hanging vertices already existed, seems like this was intended but left out of the implementation.

Now the only substantive difference between cvfdutil.gridlist_to_disv_gridprops and the Lgr approach is that the former includes "wraparound" vertices, which MF6 considers unnecessary.

Noticed this in MODFLOW-ORG/modflow6-examples#319 while switching to the Lgr approach.

Some of the test code here will be very short-lived given #2667 but I'm including it just to bake it into the git log for the benefit of any future forensics

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.7%. Comparing base (556c088) to head (2259125).
⚠️ Report is 98 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2672      +/-   ##
===========================================
+ Coverage     55.5%    72.7%   +17.1%     
===========================================
  Files          644      667      +23     
  Lines       124135   129574    +5439     
===========================================
+ Hits         68947    94222   +25275     
+ Misses       55188    35352   -19836     

Files with missing lines	Coverage Δ
flopy/utils/cvfdutil.py	84.0% <ø> (+1.5%)	⬆️
flopy/utils/lgrutil.py	97.4% <100.0%> (+0.1%)	⬆️

... and 558 files with indirect coverage changes

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