Function pullback_impl 

fn pullback_impl<const N: usize>(
    family: Family,
    label: WaveLabel,
    args: [Complex; 6],
    helicity: bool,
    cotangent: [Complex; 3],
) -> Result<[Complex; 6]>

vector_wave_pullback with derivatives in the first N arguments, the ones the family reads; the others get zero.