Fixes to keep Born level particle and store incoming parton information

As discussed in LHCBGAUSS-1533 and https://gitlab.cern.ch/LHCb-QEE/WmassMeasurement/issues/180, this fix marks particles from the hard process (+ISR and FSR) in Pythia to be kept in the final event record in Gauss for use in QEE analyses. In addition, it adds a flag to MCParticle to indicate if the particle is an incoming parton, which can be used for PDF reweighting. The changes have been presented also in https://indico.cern.ch/event/803966/contributions/3431683/attachments/1844188/3025026/sfarry_born_20190514.pdf