This is an implementation of the algorithm described by Adriano Moreira and Maribel Yasmina Santos: CONCAVE HULL: A K-NEAREST NEIGHBOURS APPROACH FOR THE COMPUTATION OF THE REGION OCCUPIED BY A SET OF POINTS. GRAPP 2007 - International Conference on Computer Graphics Theory and Applications; pp 61-68.
https://repositorium.sdum.uminho.pt/bitstream/1822/6429/1/ConcaveHull_ACM_MYS.pdf
With help from https://github.com/detlevn/QGIS-ConcaveHull-Plugin/blob/master/concavehull.py