Function delta1 

pub fn delta1(
    vA: &Vector2<f64>,
    vr: &Vector2<f64>,
    vp: &Vector2<f64>,
) -> Vector2<f64>

delta 1 for ri - rj

Examples:

use ginger::rootfinding::delta1;
use ginger::vector2::Vector2;

let mut vA1 = Vector2::new(1.0, 2.0);
let vri = Vector2::new(-2.0, -0.0);
let vrj = Vector2::new(4.0, -5.0);
let vd = delta1(&vA1, &vri, &vrj);
assert_eq!(vd, Vector2::new(0.2, 0.4));