Don't modify pspawn argument in stochastic_round_spawned_particle
Don't modify pspawn argument in stochastic_round_spawned_particle

There's no need to return the fractional part of pspawn to the calling
routine (it's not used and I can't see why it would be needed).  It's
safer not to modify arguments to functions in place (yay purity) and
allows for expressions to be used as the arguments.

hande-qmc/hande,hande-qmc/hande,hande-qmc/hande,ruthfranklin/hande,hande-qmc/hande,hande-qmc/hande