(feature) add anisotropic meshing to LearnerND

like this:

from this source

Still need to figure out whether de stretching should depend on the derivative or the second derivative

Nice idea!

I think its the derivative: imagine plotting a flat plane at an angle. To get the same triangles at different tilt angles we need to stretch differently, but in all cases the second derivative is 0

Nice reference. I have thought about particle-based methods, but decided in favor of triangulation because I thought it is simpler when we don't have a luxury of being able to query the function at will.

With having the triangulation we should be able to do just that using the transform argument to the triangulation.

I agree that particle based solutions are not very useful for adaptive. I just figured this image to be a perfect example of what we want to achieve.

added LearnerND label

mentioned in merge request !90