Introduce support for PGun simulations.

This is needed for new requests by IFT.

Note that while PGuns may be processable for Run2 Sim10 it may not be supported in the new Run3 event model yet.

Also need to check for Sim09 as the PV smearing was moved from Brunel to Gauss and we need to check how it works.

This is related to mc-productions-setup#12 were there is some discussion on check needed as needed for input to this