Refactor code for adding to spawn store into procedures.
Refactor code for adding to spawn store into procedures.

Each create_spawned_particle* procedure used (essentially) the same code
to actually add a particle (if it was going to!) to the spawned store.
This is wasteful and annoying to change.  Refactor the repeated code
into separate subroutines so the code only occurs twice rather than
N times!

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