pub fn first_derivative<G, T: DualNum<F>, F>(g: G, x: T) -> (T, T)where G: FnOnce(Dual<T, F>) -> Dual<T, F>,
Calculate the first derivative of a scalar function.
let (f, df) = first_derivative(|x| x.powi(2), 5.0); assert_eq!(f, 25.0); assert_eq!(df, 10.0);