Function fang_oost::get_density_x

pub fn get_density_x<T>(
    num_x: usize, 
    num_u: usize, 
    x_min: f64, 
    x_max: f64, 
    fn_inv: T
) -> impl IndexedParallelIterator<Item = f64> where
    T: Fn(&Complex<f64>) -> Complex<f64> + Sync + Send,