Return the crossproduct between vec_a and vec_b.

def cross_v3(vec_a, vec_b):
    """Return the crossproduct between vec_a and vec_b."""

    return Vec3(vec_a.y * vec_b.z - vec_a.z * vec_b.y,
                vec_a.z * vec_b.x - vec_a.x * vec_b.z,
                vec_a.x * vec_b.y - vec_a.y * vec_b.x)