HepMcParticleLink constructor syntax clean-up.
Avoid using 0 in combination with HepMcParticleLink::IS_EVENTNUM in constructor calls as this is not properly supported. It is more correct to directly use 0 with HepMcParticleLink::IS_POSITION to indicate that the link is to the first GenEvent in the McEventCollection.
(Relates to ATLASSIM-6999)