Remove the state and RHS from the models

[Sbte]

The models don't have an actual state. They just have some functions that take a state as input (at least mathematically). Removing the state and RHS from the models would make this more clear.

It may still be useful to have an initial state that can be obtained from the model, just so we know the initial state is valid, and so we can more easily load a state from a file.

[Sbte]

Problem with this is that the models other than Ocean also have this and use it more prominently, and that it is used for saving/loading states from files (also legacy Fortran files). I'm not sure if it's worth it to restructure this, even though it is probably safer and more correct.