WIP: Transformer is a MultiTransformer

While attempting to design a base class for the LHCb algorithms that link reconstructed to MC objects, I ran into the fact that Transformer and MultiTransformer only share their base class. Since it would be convenient to only have to define one type that inherits from Transformer, I attempted to integrate MultiTransformer back into Transformer. The merge request (to the master branch of my own fork) is meant to discuss the usefulness of this attempt.

Since the previous implementation of MultiTransformer was a specialization, and the implementation of Transformer is, I couldn't alias MultiTransformer using a using statement. The #define trick is very ugly.

Please feel free to comment @graven @rmatev @hegner @clemenci @sponce.