Add support for bagging

[LorenzBuehmann]

As DL-Learner contains several different learning algorithms, support for bagging[1] could be an interesting feature. We still would have to figure out how to aggregate several CEs...

[1] An Empirical Evaluation of Bagging in Inductive Logic Programming. Dutra et al. Proceedings of the Twelfth International Conference on Inductive Logic Programming, pp. 48--65, Sydney, Australia.