isDone() {
    return this.x < -this.xThreshold || this.x > this.xThreshold ||
        this.theta < -this.thetaThreshold || this.theta > this.thetaThreshold;
  }
Determine whether this simulation is done.

A simulation is done when `x` (position of the cart) goes out of bound
or when `theta` (angle of the pole) goes out of bound.

@returns {bool} Whether the simulation is done.