Draft: add a converter from RecVertex to AllenPVs for particle gun simulations

With particle gun simulation, the usual PV reconstruction won't work as there are no real PVs simulated. Instead a PV(RecVertex) converted/smeared from MCVertex is provided by Boole. This PV needs to be converted to Allen PVs replacing the PV reconstruction.

Part of the set: LHCb!4634 !1716 Moore!3657