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

[wpbonelli]

Improve the shared face finding algorithm for HFB (Horizontal Flow Barrier) plotting. Factor out @jlarsen-usgs's index-based approach from #2671 into reusable function(s) and use it instead of floating point comparisons as before. Should be faster.

I moved the new face-related functions to a new module faceutil.py instead of plotutil.py as they are used not only for plotting but for export, and may be useful in other cases. 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.

I also changed the name of is_vertical_barrier -> is_vertical and inverted the semantics so it works on faces, not barriers (a face between horizontally adjacent cells is a vertical polygon, and a face between vertically adjacent cells is a horizontal polygon, but we call the former a "horizontal" flow barrier and the latter a "vertical" flow barrier). I figure writing these utils in terms of faces for the general case makes sense, and just interpreting the results for the specific case of HFBs.

This PR builds on

• feat(GeoDataFrame support): geopandas support for shapefile exporting #2671's shared face finding approach
• 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

Some ad hoc cell ID -> node number conversion utilities are removed and get_node() used instead.

I am classifying all this a refactor though it introduces a new utility module, as the new utils are in service of the refactor.

[codecov]

Codecov Report

❌ Patch coverage is 63.51351% with 54 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.3%. Comparing base (556c088) to head (663aceb).
⚠️ Report is 122 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/utils/faceutil.py	62.5%	52 Missing ⚠️
flopy/mf6/mfpackage.py	0.0%	1 Missing ⚠️
flopy/modflow/mfhfb.py	0.0%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2682      +/-   ##
===========================================
+ Coverage     55.5%    72.3%   +16.8%     
===========================================
  Files          644      669      +25     
  Lines       124135   130180    +6045     
===========================================
+ Hits         68947    94221   +25274     
+ Misses       55188    35959   -19229     

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/mf6/mfpackage.py	67.3% <0.0%> (-14.3%)	⬇️
flopy/modflow/mfhfb.py	84.2% <0.0%> (-0.3%)	⬇️
flopy/utils/faceutil.py	62.5% <62.5%> (ø)

... and 559 files with indirect coverage changes

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