consider changing rhs() interface in integrator to pass y instead of integrator type

[zingale]

Right now, we pass the full integrator type (like vode_t) to the rhs() wrapper and then copy vode_t integrator_state.y() into the burn_t state to do the burn. But some integrators (like the RKC) want to pass in different states that are not always in the .y() slot in the integrator type.

We should consider changing the wrappers to explicitly pass in the state, y() instead of the entire integration state. I think that this is very straightforward.