feat(dis2d): limited support for a 2D structured grid (for overland flow)

[langevin-usgs]

• This first PR doesn't really do anything, except define model shapes and cellids for dis2d
• A DIS2D grid does no have layers or a top array, and botm and idomain have shape (nrow, ncol)
• Also considering DISV2D (no layers)
• Also considering renaming of DISL to DISV1D for 1D line network described by vertices
• No testing yet
• A modelgrid for this discretization type has not been addressed yet
• Unclear what type of plotting and export to support

[codecov]

Codecov Report

Attention: Patch coverage is 36.66667% with 19 lines in your changes are missing coverage. Please review.

Project coverage is 73.1%. Comparing base (ed262db) to head (908dbbf).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2131     +/-   ##
=========================================
- Coverage     73.1%   73.1%   -0.1%     
=========================================
  Files          260     260             
  Lines        59411   59439     +28     
=========================================
+ Hits         43445   43454      +9     
- Misses       15966   15985     +19     

Files	Coverage Δ
flopy/mf6/utils/mfenums.py	100.0% <100.0%> (ø)
flopy/mf6/coordinates/modeldimensions.py	78.5% <0.0%> (-0.5%)	⬇️
flopy/mf6/coordinates/modelgrid.py	37.1% <37.0%> (-0.7%)	⬇️

... and 1 file with indirect coverage changes