Function authalic_lat 

pub fn authalic_lat(
    phi: f64,
    sinphi: f64,
    cosphi: f64,
    apa: &[f64],
    proj: &Proj,
    qp: f64,
) -> f64

Computes authalic latitude from the geographic latitude. qp is q at phi=90deg, i.e. qp = pj_authalic_lat_q(1, e, one_es)