refactor(plot): better shared face finding for HFB plotting

[wpbonelli]

Improve the shared face finding algorithm for HFB (Horizontal Flow Barrier) plotting. Factors out @jlarsen-usgs's index-based approach from #2671 into reusable functions. The index-based approach avoids floating point comparisons and will be faster than the previous approach. I also moved the new face-related functions to a new module faceutil.py instead of plotutil.py so #2671 can reuse them for export, and in case they are generally useful. I considered putting them in gridutil.py but that is used by the grid classes, while the face functions use the grid classes, so it would have created a circular import.

This PR takes advantage of

• feat(grid): add get_node() method to all grid types #2680 which gives all grid types a consistent get_node() method
• feat(grid): add ihc to UnstructuredGrid #2681 which adds an ihc init parameter/attribute to UnstructuredGrid

[codecov]

Codecov Report

❌ Patch coverage is 79.66102% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.7%. Comparing base (556c088) to head (6441db7).
⚠️ Report is 108 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/utils/faceutil.py	78.3%	24 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2682      +/-   ##
===========================================
+ Coverage     55.5%    72.7%   +17.2%     
===========================================
  Files          644      669      +25     
  Lines       124135   129418    +5283     
===========================================
+ Hits         68947    94150   +25203     
+ Misses       55188    35268   -19920     

Files with missing lines	Coverage Δ
flopy/plot/crosssection.py	64.5% <100.0%> (-2.6%)	⬇️
flopy/plot/map.py	81.5% <100.0%> (-0.6%)	⬇️
flopy/plot/plotutil.py	51.2% <100.0%> (-5.6%)	⬇️
flopy/utils/faceutil.py	78.3% <78.3%> (ø)

... and 560 files with indirect coverage changes

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