Function second_best_loss 

pub fn second_best_loss<S, A, F>(
    actions: &[A],
    state_estimate: &S,
    best_idx: usize,
    loss_fn: F,
) -> f64
where
    S: State,
    A: Action,
    F: Fn(&A, &S) -> f64,

Find the second-best loss (for computing loss_avoided).