[Question] Particle Energy

@browles Is there any plan to implement events triggered by the `Particle` itself? Right now if I want to stop requesting updates for the `PhysicsEngine` I need to listen for the `poststep` and then manualy calculate the energy for each `Particle` using the formula: `0.5 * this.mass * this.velocity.normSquared()` .. ?

Is there a plan to reimplement the energy calculations?

I think stopping and starting the `PhysicsEngine` is better than always letting it run .. (?)