Resolve "(Learner1D) add possibility to use the direct neighbors in the loss"

Closes #119 (closed)

Currently works for one R^1 \to R^1

Still have to make it work for R^1 \to R^N

Also performance is actually quite good. As in: the learner slows down about 1.5 times. Going (on my laptop) from 3 seconds per 1000 points to 4.5 seconds per 1000 points.

Which, I believe, will be more than compensated by the fact that the chosen points are generally better

docs/source/reference/adaptive.learner.learner1D.rst

@@ -12,3 +12,9 @@ Custom loss functions
 .. autofunction:: adaptive.learner.learner1D.default_loss
 
 .. autofunction:: adaptive.learner.learner1D.uniform_loss
+
+.. autofunction:: adaptive.learner.learner1D.uses_nth_neighbors
+
+.. autofunction:: adaptive.learner.learner1D.triangle_loss
+
+.. autofunction:: adaptive.learner.learner1D.get_curvature_loss