Function regular_harmonic 

pub(crate) fn regular_harmonic<const N: usize>(
    order: i32,
    transverse: Jet<N>,
    r: [Jet<N>; 3],
    kz: Jet<N>,
) -> Jet<N>

Regular J_m harmonic as a Cartesian power series, for |k_rho rho| below SERIES_RADIUS. Factoring the azimuthal polynomial keeps derivatives regular on the cylinder axis.