add sibyll process NuclearInteraction

Working on the integration of nuclei in the SIBYLL interface, I came to the realization that it does not make sense to handle both with the same process. Everything that is already implemented for hadrons would have to be equipped with a if IsNucleus ... else.... The reason is of course that nuclear interactions are not really integrated in SIBYLL in the first place, since it uses the semi-superposition approach.

For the start I copied the existing structure of sibyll::Interaction. In the long run we would want an independent process that can be equipped with any implementation of the hadronic interactions, e.g. something like NuclearInteraction<sibyll::Interaction>. But I think this may be solved at a later time.

related to #65 (closed) and #90 (closed)