Function multiply_outer 

pub fn multiply_outer<T>(
    a: &Array<T, Ix1>,
    b: &Array<T, Ix1>,
) -> Result<Array<T, IxDyn>, FerrayError>
where
    T: Element + Mul<Output = T> + Copy,

Outer product: multiply_outer(a, b)[i, j] = a[i] * b[j].

Equivalent to np.multiply.outer(a, b). Delegates to the generic crate::ufunc_methods::outer with the * kernel.

AC-3: multiply_outer produces correct outer product.