PullBackString2D = function(a, b, strength) {
            VerletSpring2D.apply(this, [a, b, 0, strength]);
            a.lock();
            this.setRestLength(0.5);
          }
Creates a pullback spring (default restlength=0.5) between 2 particles and
locks the first one given at the current position. The spring is only
enforced if the current length of the spring exceeds the rest length. This
behaviour is the opposite to the {@link VerletMinDistanceSpring2D}.