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[bot]]

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

```diff @@ 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](https://app.codecov.io/gh/modflowpy/flopy/pull/2131?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy) | Coverage Δ | | |---|---|---| | [flopy/mf6/utils/mfenums.py](https://app.codecov.io/gh/modflowpy/flopy/pull/2131?src=pr&el=tree&filepath=flopy%2Fmf6%2Futils%2Fmfenums.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy#diff-ZmxvcHkvbWY2L3V0aWxzL21mZW51bXMucHk=) | `100.0% <100.0%> (ø)` | | | [flopy/mf6/coordinates/modeldimensions.py](https://app.codecov.io/gh/modflowpy/flopy/pull/2131?src=pr&el=tree&filepath=flopy%2Fmf6%2Fcoordinates%2Fmodeldimensions.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy#diff-ZmxvcHkvbWY2L2Nvb3JkaW5hdGVzL21vZGVsZGltZW5zaW9ucy5weQ==) | `78.5% <0.0%> (-0.5%)` | :arrow_down: | | [flopy/mf6/coordinates/modelgrid.py](https://app.codecov.io/gh/modflowpy/flopy/pull/2131?src=pr&el=tree&filepath=flopy%2Fmf6%2Fcoordinates%2Fmodelgrid.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy#diff-ZmxvcHkvbWY2L2Nvb3JkaW5hdGVzL21vZGVsZ3JpZC5weQ==) | `37.1% <37.0%> (-0.7%)` | :arrow_down: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/modflowpy/flopy/pull/2131/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy)