pub fn authalic_lat_q(sinphi: f64, proj: &Proj) -> f64
Computes coefficient q such that authalic_latitude = beta = asin(q / qp) where qp is q at phi=90deg, i.e. qp = authalic_lat_q(1, e, one_es) Cf Snyder (3-11) and (3-12)