Expose state derivative yp_ for signals

Summary

For a given signal of state y wired into a model, that model should also have access to the derivative yp

Rationale

Many models read external derivatives

Description

Expose state and state derivative for a given signal. Concerns/questions of mine:

• This only makes sense for differential variables, so we should use tag_ to validate, I think
• This might possibly add some non-trivial inclusion of $\alpha$ directly into the equations? unsure

Additional information

@nkoukpaizan I would like to know your thoughts on this.