refactor(grid): consistent API for get_cell_vertices

[wpbonelli]

Previously the Grid.get_cell_vertices() method looked/worked different depending on the grid type. Make it consistent across all three grid types by supporting both cellid and node keyword arguments and being lenient about what's passed to cellid i.e. accept a node number, a fully-sized tuple, or an "abbreviated" tuple lacking layer index. Cell ID and node number are conceptually distinct but til now VertexGrid and UnstructuredGrid methods had a single parameter cellid supporting (for vertex) or expecting (for unstructured) a node number, which we don't want to break. And continue also supporting i and j for StructuredGrid.

Noticed these inconsistencies when working on #2677

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.7%. Comparing base (556c088) to head (3020a90).
⚠️ Report is 103 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/discretization/structuredgrid.py	68.4%	6 Missing ⚠️
flopy/discretization/vertexgrid.py	73.6%	5 Missing ⚠️
flopy/discretization/unstructuredgrid.py	72.7%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2678      +/-   ##
===========================================
+ Coverage     55.5%    72.7%   +17.1%     
===========================================
  Files          644      667      +23     
  Lines       124135   129706    +5571     
===========================================
+ Hits         68947    94341   +25394     
+ Misses       55188    35365   -19823     

Files with missing lines	Coverage Δ
flopy/discretization/unstructuredgrid.py	76.8% <72.7%> (-4.7%)	⬇️
flopy/discretization/vertexgrid.py	80.1% <73.6%> (-3.5%)	⬇️
flopy/discretization/structuredgrid.py	53.1% <68.4%> (+5.6%)	⬆️

... and 557 files with indirect coverage changes

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[wpbonelli]

Cell ID and node number are conceptually distinct but up to now the VertexGrid and UnstructuredGrid versions of this method had a single parameter cellid supporting (for vertex) or expecting (for unstructured) a node number. So I think for consistency we can allow passing node number to cellid for StructuredGrid too.