function Gravity3D(source, targets, options) {
    this.source = source || null;
    Force.call(this, targets, options);
}
An inverse square force dependent on the masses of the source and targets.

@class Gravity3D
@extends Force
@param {Particle} source The optional source of the attraction field.
@param {Particle[]} targets The targets to affect.
@param {Object} options The options hash.