feat(grid): add get_node() method to all grid types

[wpbonelli]

Add a get_node() method to VertexGrid and UnstructuredGrid, consistent with StructuredGrid, for cell ID -> node number conversions. Rename the lrc_list parameter on StructuredGrid.get_node() to cellids and deprecate lrc_list. Add an optional node2d parameter to determine whether to get the 2D or 3D node number, useful for structured and vertex grids, ignored for unstructured.

The cell ID -> node number conversion was done ad hoc in a few places in the codebase, this allows some simplification. Maybe in 4.x we can standardize the signature and pull an abstract method into the base Grid class.

[codecov]

Codecov Report

❌ Patch coverage is 94.44444% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.8%. Comparing base (556c088) to head (018e548).
⚠️ Report is 107 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/discretization/structuredgrid.py	86.6%	2 Missing ⚠️
flopy/discretization/unstructuredgrid.py	93.7%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2680      +/-   ##
===========================================
+ Coverage     55.5%    72.8%   +17.2%     
===========================================
  Files          644      667      +23     
  Lines       124135   130158    +6023     
===========================================
+ Hits         68947    94778   +25831     
+ Misses       55188    35380   -19808     

Files with missing lines	Coverage Δ
flopy/discretization/vertexgrid.py	81.0% <100.0%> (-2.7%)	⬇️
flopy/export/utils.py	54.3% <100.0%> (-10.3%)	⬇️
flopy/plot/crosssection.py	64.6% <100.0%> (-2.5%)	⬇️
flopy/utils/binaryfile/__init__.py	84.2% <100.0%> (-0.3%)	⬇️
flopy/discretization/unstructuredgrid.py	81.9% <93.7%> (+0.4%)	⬆️
flopy/discretization/structuredgrid.py	53.4% <86.6%> (+5.9%)	⬆️

... and 555 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[wpbonelli]

discussed offline with @jlarsen-usgs