Function exp_sustained_impulse 

pub fn exp_sustained_impulse<T: SignedNumber + Float, X: Base<T> + FloatOps<T> + SignedNumberOps<T> + NumberOps<T>>(
    x: X,
    f: X,
    k: X,
) -> X

returns an exponential sustained impulse (y position on a graph for x); control on the width of attack with k and release with f