Allow direct process model

[remipch]

By reading your library, I guess it was practical for you to model your specific system by writing derivative formulas.

Then, you implemented Integrator classes to compute process model from the derivative model.

In our case, it was convenient to write the direct process model without writing explicit derivative formulas.

This PR consists of allowing to implement StateVector::process_model() method directly without needing to implement StateVector::derivative() method.