Distinguish between state space models with and without "sigma" parameters.

[steve-the-bayesian]

Some model families leave "sigma" implicit. This has implications for some state models which want to use the "sigma" parameter from the host model.

The solution is to separate the AddState method in the state model factories into two parts. AddStateToSigmaParameterModel and AddState. The first checks for individual methods that need "sigma" and defaults to the second if not found.