Function fidelity 

pub fn fidelity<R>(
    ideal: &HilbertState<R>,
    actual: &HilbertState<R>,
) -> PropagatingEffect<R>
where
    R: RealField + FromPrimitive + Sum + Default + Debug,

Causal wrapper for mechanics::fidelity_kernel.

Like born_probability, the value channel carries the plain real fidelity in [0, 1].