𝛘-hull implementation

[Jeroen Bloemscheer]

Created a pull request for a 𝛘-hull implementation which is a simple algrorithm for creating a concave hull with all points connected, based on this research paper.

At the time of writing all unit test seem to pass but had to tweak the yml a bit because the unit test took more than 10 minutes to be processed by travis. Not sure why the same build byairbreatherworked in less than 1 minute and the same code by me took more than 10 minutes.

Have added a unit test based on example given on turf.js with the same algorithm.

In the template have not yet checked "I have verified that I am following the existing coding patterns and practice as demonstrated in the repository." are there some guidelines in this community so that I can check this myself or could someone look at my code :)

[Jeroen Bloemscheer]

PR build timeouts only seem to occur on new participants. Changed yaml on tensing group and performance issue resurfaces.