function Spring(source, targets, options) {
    this.source = source || null;
    Force.call(this, targets, options);
}
A force that accelerates a Particle towards a specific anchor point. Can be anchored to
a Vec3 or another source Particle.

@class Spring
@extends Force
@param {Particle} source The optional source of the spring.
@param {Particle[]} targets The targets to affect.
@param {Object} options The options hash.