fix all implementations of "predict*" that imply that only one single instance was given

[ChristianSch]

The notation and docs hint at this, but we don't do it yet. The doc/notation implies that a number of instances was given, for which each of the instances should be getting a predicted target. Currently we just imply that X is an instance given as a vector and run predictions on that.

[ChristianSch]

This should be done, however there should be tests for that.