Pythia8 converter - fix for beam particle identification

The converter for Pythia8 in HepMC3 produces different events compared to the HepMC2 converter found in Pythia8 itself (sometimes drastically lower number of particles). This seems to be caused by how the beam particles are identified. The particle with index 0 (cc @philten) is used to define the event and thus the beam particles should be identified as IDs 1 and 2 (as is done for HepMC2).