ParticleCut doesn't affect "non-secondary generating" processes

Particles suffering only energy losses during a step won't be deleted by the ParticleCut process if they drop below threshold. This puts UrQMD in trouble when the final energy is below what UrQMD can handle. Therefore I am forced to put a threshold for the ParticleCut a few GeV higher than actually necessary for UrQMD.

The problem is that ParticleCut is a SecondaryProcess but there are no secondaries.