Add a meta trainer

[ASalvail]

Working with the lib, I've come to the conclusion that it might be useful to have a meta trainer. This trainer would be used to do a series of experiments (k-fold, hyper parameters seach, etc).

This would be particularly useful coupled with the tasks at initialization and at the end (producing reports, etc.)
Instead of epochs, we could add another level : experiments.
Of course, per epochs and updates tasks would be directly transferred to its children.

Any other ideas that'd be useful?