Function add_pair_gradient 

pub(crate) fn add_pair_gradient(
    result: &mut ExpansionGradient,
    [p, q, pol]: [usize; 3],
    cot: Complex,
    position: [Complex; 3],
    k: Complex,
)

Pull the cotangent cot of an entry depending on the displacement destination[p] - source[q] (derivative position) and the medium wavenumber of polarization pol (derivative k) back into result.