(LearnerND) Make some functions faster using Cython

Closes #80

This speeds up the learner with ~30% I believe.

I am aware that currently a great amount of time is going into the triangulation class. And I think that it can become a gazillion times faster if it would be fully translated into cython (or C++). However, I am not sure if we would want to go this route, now. As it would be a great deal of work. And I suppose it would be beneficial to make this change only if we are certain that only minor parts of the triangulation will change.