Create SIBYLL process that handles everything including nuclei

Our interfaces of both, EPOS and QGSJet, make no distinction between single hadron and nuclei projectiles. For nuclear showers with SIBYLL, however, we need to have two processes in the process sequence. I propose to implement an additional (meaning the two existing ones stay as they are since there are good reasons to have them separate) lightweight wrapper to combine these two into a single one. This would make a few things easier, e.g. for a dynamic selection of the interaction model at runtime.