Internal function to sample and set up data structure for batch migration
batch_migration_stage(SPN_P, rates, move, prob, stage, tmax)
a set of Petri net places
a vector of rates for each node
a movement matrix (where do the batches go?)
the probability vector for each individual moving in each batch
the life stage (one of 'ELP', 'F', 'M')
maximum simulation time