1D: fix the rare case where the right boundary point exists before the left bound

Closes #94

See merge request !95

It's now done by Gitlab itself using kwant-bot's account on
Github.

(LearnerND) Evaluate fewer circumspheres

See merge request !91

refactor LearnerND._ask to be more readable (and faster)

Closes #72

See merge request !88

it prunes some circumcircles faster
it results in:
~20% faster in 2d
~40% faster in 3d

(LearnerND) Improve the new point selection by choosing the
simplex center, or center of the longest edge, depending
on the "quality" of the simplex. Also change the default
loss to be the generalization of the Learner1D default loss.


Closes #76 and #77

See merge request !86