Delete prim from simulation and keep the simulation running

@apx103 the conclusion is that, unfortunately, it is not possible to delete objects on the fly. You will need to call stop on the simulation context (e.g. world.stop()), delete the prims, then start simulation again. The reason for this is that right now when deleting a prim it is not possible to fully synchronize the event with the physics engine. To protect against faulty behaviors, when deleting a prim the simulation context is invalidated.