Function add_agent 

pub fn add_agent<S, A, Store, Props, R, Sch>(
    model: &mut StandardModel<S, A, Store, Props, R, Sch>,
    agent: A,
) -> Result<(), InteractionError<S::Error>>
where
    A: PositionedAgent,
    S: SpaceInteraction<A>,
    Store: AgentStore<A>,
    R: RngCore,
    Sch: Scheduler<StandardModel<S, A, Store, Props, R, Sch>>,

Add a positioned agent to both the store and the space.

Returns InteractionError::DuplicateId if an agent with the same ID already exists, or InteractionError::Space if the space rejects the position.