Simulates a two-stage process where all individuals
in a given source_state sample whether to leave or not with probability
rate; those who leave go to one of the destination_states with
probabilities contained in the vector destination_probabilities.
fixed_probability_multinomial_process(
  variable,
  source_state,
  destination_states,
  rate,
  destination_probabilities
)a CategoricalVariable object.
a string representing the source state.
a vector of strings representing the destination states.
probability of individuals in source state to leave.
probability vector of destination states.
a function which can be passed as a process to simulation_loop.