Add LearnerND that does not interpolate the values of pending points

it has some basic functionality now, there are still some things I am going to fix before we can actually use it:

TODO's:

• add LearnerND
• make a generic loss function that zooms in on deviations
• make a better generic loss function (separate issue)
• allow asking multiple points in one go
  • take pending points into account when choosing a new point
• come up with a better choose_point_in_simplex (separate issue)
• loss_per_simplex should only take a single simplex as input, not the entire triangulation
• make Learner2D a child class of LearnerND (like a special case of the general case)
• Performance
  • do not recompute triangulation so often
  • do not recompute losses for simplices of which you already know the loss
• Triangulation: flip edges when it's beneficial
• add tests for the LearnerND
• add plot_slice functionality for 2D plots as well
• add documentation (both user documentation as well as developer documentation) [currently docstrings and comments]
• add 3d surface plot (separate issue)
• add 3d crossection plot
• scale Y-values to unit distance as well
• allow for N-dimensional output (partially resolved, see #79 (closed))

Closes #52 (closed) and adds a LearnerND