Function angular_distance 

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

Returns the angular separation (in radians) between two spherical points.

Uses the Vincenty formula for numerical stability at all separations.

use sphereql_core::{SphericalPoint, angular_distance};

let a = SphericalPoint::new_unchecked(1.0, 0.0, 0.0);
let b = SphericalPoint::new_unchecked(1.0, 0.0, std::f64::consts::FRAC_PI_2);
let dist = angular_distance(&a, &b);
assert!((dist - std::f64::consts::FRAC_PI_2).abs() < 1e-10);