fix(cvfdutil): polygon area and centroid calculations now use shapely

[langevin-usgs]

• Changed area_of_polygon and centroid_of_polygon to use shapely calculations
• Close #2150
• Added test, which uses a polygon that was failing with the previous implementation

Decided to use shapely calculations instead of our own flopy calculations. This should much more robust for real-world models, such as the one described in #2150.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 62.50000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 71.0%. Comparing base (029a4e1) to head (a9906c1). Report is 2 commits behind head on develop.

:exclamation: Current head a9906c1 differs from pull request most recent head c942148. Consider uploading reports for the commit c942148 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #2165 +/- ## ========================================= + Coverage 68.3% 71.0% +2.7% ========================================= Files 260 260 Lines 57947 58128 +181 ========================================= + Hits 39611 41318 +1707 + Misses 18336 16810 -1526 ``` | [Files](https://app.codecov.io/gh/modflowpy/flopy/pull/2165?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/utils/cvfdutil.py](https://app.codecov.io/gh/modflowpy/flopy/pull/2165?src=pr&el=tree&filepath=flopy%2Futils%2Fcvfdutil.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy#diff-ZmxvcHkvdXRpbHMvY3ZmZHV0aWwucHk=) | `80.7% <62.5%> (-2.6%)` | :arrow_down: | ... and [178 files with indirect coverage changes](https://app.codecov.io/gh/modflowpy/flopy/pull/2165/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy)