Function argmin::testfunctions::sphere_derivative

pub fn sphere_derivative<T>(param: &[T]) -> Vec<T> where
    T: FromPrimitive + Float, 

Derivative of sphere test function

Defined as

f(x_1, x_2, ..., x_n) = (2 * x_1, 2 * x_2, ... 2 * x_n)

where x_i \in (-\infty, \infty) and n > 0.