Remove _choose_points

Currently there is choose_points, which just calls _choose_points and adds (or not) the data to self.data.

There are several examples (BalancingLearner and 2DLearner) where it makes most sense to implement choose_points directly.

Even in this case we still need to

def _choose_points(self, n):
    pass

because it is labelled as an abstract method.

mentioned in commit 1d4d7a28

mentioned in commit 6e23fa06

mentioned in commit a8866d4a

closed via commit a8866d4a

closed via commit cb3bdfe2

closed via merge request !7 (merged)

mentioned in commit cb3bdfe2