possible errors when naming keys in data

[edublancas]

When using Pipeline, the user sends a dictionary in the load method, the keys for this dictionary are arbitrary and this does not cause any trouble.

But when using SKExperiment and trying to train an experiment, the MetaEstimator assumes certain naming conventions for the keys. I should make this explicit. Probably raising and exception in skpipeline.load when data does not have the appropiate keys