Merging sibyll::Interaction and sibyll::NuclearInteraction

I think sibyll::Interaction and sibyll::NuclearInteraction should be merged. Separating these two only makes sense if the two could be used independently of each other, but sibyll::NuclearInteraction has to hold a reference to sibyll::Interaction, which means that it cannot be used without sibyll::Interaction in the process sequence.

I think this would also make Sibyll run faster, because some if branching can be avoided when both are joined.