function Force(targets, options) {
    if (targets) {
        if (targets instanceof Array) this.targets = targets;
        else this.targets = [targets];
    }
    else this.targets = [];

    options = options || {};
    this.setOptions(options);

    this._ID = _ID++;
}
Abstract force manager to apply forces to targets.

@class Force
@virtual
@param {Particle[]} targets The targets of the force.
@param {Object} options The options hash.