Provide Physics Stepper for non-Amethyst users

As far as I know, the old stepper was not integrated in this version of the library.

The old one can be found here: https://github.com/distransient/nphysics-ecs-dumb/blob/master/src/systems/physics_stepper.rs

For a lot of games, including mine, multiple steps per frame is an essential feature.

Are we supposed to externally manage the stepping, or were you planning on having this feature included in the stepper system?