updates(model_splitter.py): update UnstructuredGrid support

[jlarsen-usgs]

• force remapping to be calculated from iac, ja array
• add 0 digit padding to file names
• update UnstructuredGrid, add cell2d input parameter and property method
• add clean_iverts() to UnstructuredGrid to remove duplicated vertex and invert information
• update testing for new features

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 72.50000% with 33 lines in your changes missing coverage. Please review.

Project coverage is 74.4%. Comparing base (559ae74) to head (2cd8fd8). Report is 8 commits behind head on develop.

Files	Patch %	Lines
flopy/discretization/unstructuredgrid.py	50.8%	29 Missing :warning:
flopy/mf6/utils/model_splitter.py	92.4%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #2292 +/- ## ========================================= + Coverage 73.9% 74.4% +0.4% ========================================= Files 294 294 Lines 59394 59486 +92 ========================================= + Hits 43932 44268 +336 + Misses 15462 15218 -244 ``` | [Files](https://app.codecov.io/gh/modflowpy/flopy/pull/2292?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/mfmodel.py](https://app.codecov.io/gh/modflowpy/flopy/pull/2292?src=pr&el=tree&filepath=flopy%2Fmf6%2Fmfmodel.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy#diff-ZmxvcHkvbWY2L21mbW9kZWwucHk=) | `82.8% <100.0%> (-0.2%)` | :arrow_down: | | [flopy/utils/rasters.py](https://app.codecov.io/gh/modflowpy/flopy/pull/2292?src=pr&el=tree&filepath=flopy%2Futils%2Frasters.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy#diff-ZmxvcHkvdXRpbHMvcmFzdGVycy5weQ==) | `74.2% <100.0%> (+0.3%)` | :arrow_up: | | [flopy/mf6/utils/model\_splitter.py](https://app.codecov.io/gh/modflowpy/flopy/pull/2292?src=pr&el=tree&filepath=flopy%2Fmf6%2Futils%2Fmodel_splitter.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy#diff-ZmxvcHkvbWY2L3V0aWxzL21vZGVsX3NwbGl0dGVyLnB5) | `70.5% <92.4%> (+0.7%)` | :arrow_up: | | [flopy/discretization/unstructuredgrid.py](https://app.codecov.io/gh/modflowpy/flopy/pull/2292?src=pr&el=tree&filepath=flopy%2Fdiscretization%2Funstructuredgrid.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy#diff-ZmxvcHkvZGlzY3JldGl6YXRpb24vdW5zdHJ1Y3R1cmVkZ3JpZC5weQ==) | `80.8% <50.8%> (-3.8%)` | :arrow_down: | ... and [10 files with indirect coverage changes](https://app.codecov.io/gh/modflowpy/flopy/pull/2292/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy)

[jdhughes-usgs]

@jlarsen-usgs is this ready to go in or do you have more to do?

[jlarsen-usgs]

@jdhughes-usgs It's ready to go in. I'll merge it now.