Add concave hull methods

[juliohm]

There are various methods in the literature to compute the "concave" hull of a 2D point set:

https://www.researchgate.net/publication/312373158_A_Concave_Hull_Based_Algorithm_for_Object_Shape_Reconstruction

We can use ConcaveHull.jl as a test bed, it implements the method that is based on the modification of Jarvis march that we already have implemented with k-nearest neighbors.