tick (dt) {
		const that = this,
			emitters = that.emitters,
			numEmitters = emitters.length;

		dt = dt || that.fixedTimeStep;

		if (numEmitters === 0) return;

		for (let i = 0; i < numEmitters; ++i) {
			emitters[i].tick(dt);
		}

		that._flagUpdate();
		return that;
	}
The main particle group update function. Call this once per frame.

@param  {Number} dt
@return {this}