ISF_Fatras: fix status values for secondaries produced by ISF_Fatras
Cleaned-up the setting of initial properties of ISFParticles created for secondary particles
created by ISF_Fatras. Mostly this is related to setting the status correctly and (for now)
explicitly setting the ISFParticle::id to HepMC::UNDEFINED_ID
. (In future this will be set by
the ISFTruthIncident - see !71407 (merged)).
This MR also adds some FIXME comments to be followed up in future MRs.