Function fang_oost::get_density

pub fn get_density<'a, 'b: 'a, T>(
    num_u: usize, 
    x: &'b [f64], 
    fn_inv: T
) -> impl IndexedParallelIterator<Item = f64> + 'a where
    T: Fn(&Complex<f64>) -> Complex<f64> + Sync + Send + 'a,