Function pair_tangent 

pub(crate) fn pair_tangent(
    destination: &[[f64; 3]],
    source: &[[f64; 3]],
    ks: [Complex; 2],
    [p, q, pol]: [usize; 3],
    position: [Complex; 3],
    k: Complex,
) -> Complex

Contract an entry’s local derivatives with its position and medium directions.