Implement alpha shapes for alternate flash area calculation

[deeplycloudy]

There is a nice analysis of using alpha shapes to allow for a "concave hull" to define the LMA flash area in Winn et al. (2021). Alpha shapes prune away the outer edges of the Delaunay triangulation that is used to define the convex hull, with the alpha parameter used to define a scale length beyond which edges should be pruned.

There is an alphashape library on PyPI, which will directly calculate the shape from a set of points. However, that library is meant to be a high level interface, and xlma-python already calculates the triangulation in 2D and 3D, so there is no reason to repeat that calculation a second time. It would be worthwhile to study the low-level implementation details in alphashape to see if we can adapt the calculations to use our existing triangulation, perhaps by calling a supporting subroutine instead of the high level interface in the alphashape library.