Function orient3d 

pub fn orient3d(a: Point3, b: Point3, c: Point3, d: Point3) -> f64

Compute the exact orientation of point d relative to the plane through (a, b, c).

Returns a positive value if d lies below the plane (a, b, c appear counter-clockwise when viewed from above), negative if above, zero if coplanar. This matches the convention of the robust crate.