ENH: Add grid boundary polygons

[tnatt]

PR which resolves #925 and adds a new method grid.get_boundary_polygons() for extracting boundary polygons from a grid instance.

Key notes:

• kept same signature as the surf.get_boundary_polygons() except added an optional filter_array argument for defining a specific part of the grid that the boundary should be extracted from e.g. a region or around specific conditions.
• estimates an appropriate alpha value using the xinc and yinc from the grid.
• to speed up performance the xy points are averaged over layers in the grid to reduce number of points before doing the triangulation.
• added caching of the grid.get_xyz(), grid.get_dx() and grid.get_dy() to speed up performance when the method is run multiple times in loop with different filter_array input (e.g. looping over and creating region polygons).

This new method forms the basis for a potential future method with a more simplified user-interface for extracting boundaries around unique values in a discrete property. This should be added after adding support for polygon attributes #900, so that we can have a NAME attribute that corresponds to the code name.

[codecov-commenter]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (33652a2) 80.08% compared to head (f79b42b) 80.01%. Report is 4 commits behind head on main.

Files	Patch %	Lines
src/xtgeo/grid3d/_grid_boundary.py	68.75%	7 Missing and 3 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1169 +/- ## ========================================== - Coverage 80.08% 80.01% -0.07% ========================================== Files 97 98 +1 Lines 13612 13700 +88 Branches 2186 2199 +13 ========================================== + Hits 10901 10962 +61 - Misses 1981 2006 +25 - Partials 730 732 +2 ```

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