set in steering file with:
SIM.readerParameters = {"ParticlesPerEvent":"3"}

 - use skipNEvents for now as this is the only way to provide
   a parameter to a reader
 - updated the documentation

 - adds a primary vertex for every pair particle to the event
 - requires current DD4hep HEAD with new interface:
   Geant4EventReader::readParticles(int,Vertices&, Particles& )