Function lambda12 

pub fn lambda12(
    g: &GeodGeodesic,
    sbet1: f64,
    cbet1: f64,
    dn1: f64,
    sbet2: f64,
    cbet2: f64,
    dn2: f64,
    salp1: f64,
    calp1: f64,
    slam120: f64,
    clam120: f64,
    psalp2: &mut f64,
    pcalp2: &mut f64,
    psig12: &mut f64,
    pssig1: &mut f64,
    pcsig1: &mut f64,
    pssig2: &mut f64,
    pcsig2: &mut f64,
    peps: &mut f64,
    pdomg12: &mut f64,
    diffp: bool,
    pdlam12: &mut f64,
    ca: &mut [f64],
) -> f64

Compute lambda12