ODE based stepping compatible with `jax.lax.scan`

This issue is based on f2f with @dylanhmorris a while back where we speculated about having modules for propagating continuous time dynamics in a way compatible with the basic code structure of `pyrenew` models.

https://docs.kidger.site/diffrax/usage/manual-stepping/ would do the job I suspect.

Leaving this here as a note.