WIP: (feature) add anisotropic meshing to LearnerND

Closes #74

Depends on !86 (merged) and #80 and therefore it has the corresponding branches included as well

Still has a few to-do's:

• let LearnerND.ip() make use of our triangulation rather than building a new one
• make it work in arbitrary dimensions
• verify that it is beneficial
• let the user configure the parameters (maximum stretch factor and number of points to take into account) Use one simplex and it's neighbours
• make test pass
  • add rtree as install requirement No more RTree anymore :)
  • raise exception if anistropic=True and rtree not installed, pass if anisotropic=False
• refactor code to be human-readable
• let's make it fast :) #80

As it seems it doesn't work that well with the ring, since this ring has a relative low average gradient and very high second derivative. So maybe this second derivative might be a more useful property to determine the

Sneak peek: