ENH: Use shapely to create boundary polygons

[tnatt]

PR to change the _create_boundary_polygons() function to use shapely functions to connect points into polygons.

The polygons extracted using shapely have identical geometries as the polygons extracted by the old method.

However the old method was creating duplicate points along the polygons, by using shapely these are now dropped except for the start/end point of the polygon which connects the polygon together.

• Meaning a polygon that had 10 items now have 10/2 + 1 = 6 points as seen below

To account for duplicate points no longer present in the polygons, some adjustments was needed to the tests.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (483b753) 80.29% compared to head (d0dbb32) 80.27%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1140 +/- ## ========================================== - Coverage 80.29% 80.27% -0.02% ========================================== Files 97 97 Lines 13560 13548 -12 Branches 2174 2172 -2 ========================================== - Hits 10888 10876 -12 Misses 1951 1951 Partials 721 721 ```

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