Function random_agent 

pub fn random_agent<'a, S, A, Store, Props, R, Sch>(
    model: &'a mut StandardModel<S, A, Store, Props, R, Sch>,
) -> Option<Ref<'a, A>>
where
    A: Agent,
    S: Space,
    Store: AgentStore<A>,
    R: RngCore,
    Sch: Scheduler<StandardModel<S, A, Store, Props, R, Sch>>,

Pick a random agent reference from the model.

Returns None if the agent store is empty.