Function spherical_excess 

pub fn spherical_excess(
    a: &SphericalPoint,
    b: &SphericalPoint,
    c: &SphericalPoint,
) -> f64

Spherical excess (= area) of a triangle on S² with vertices a, b, c.

Uses L’Huilier’s theorem: E = 4·arctan(√[tan(s/2)·tan((s−a)/2)·tan((s−b)/2)·tan((s−c)/2)])