1) Added a prod_ident parameter, which is useful if you use multiple mc productions in one dataset.

E.g. if you use the 1-5GeV & the 3-100GeV ORCA 2016 MC, it is not possible to distinguish the >3GeV events from the lowE prod from the events of the highE prod.

2) Now, always a fixed random seed is used. This is needed, if you use multiple input datasets and you want to throw away a fixed portion of events.