Parameter collections

Provide parameter collection pattern for easier model definition. This commit also addresses #227

We may need to make some specialized code to account for some recent changes. We deprecated BetaParameter and DirichletParameter with some of the changes for GeneralizedEM.  Previously, we made a special factor for parameter elements (Beta and Dirichlet).  However, with the particle BP code, we can now create factors for continuous elements in general. The old behavior may no longer be correct if you are using atomic beta and dirichlet elements and not using them in a EM model.