Models: more modular arrangement of files (Easier to add more models)

[drbenvincent]

At the moment, it is relatively trivial to add new models. However, the files we need to add (or change) are somewhat scattered, so it would be nice to have a folder you can just drop in and this contains everything needed for a new model. This would be good as it will provide a clear template for people to implement their own models.

Currently each model needs:

• a concrete implementation of the Model class (in the ModelClasses folder)
• corresponding JAGS and STAN models (in the models folder)
• currently we also need a *clusters.m plot function. This can be done away with.
• we have discount functions (concrete DeterministicFunction)

Things to change:

• [x] put each model in their own folder.
• [x] move the relevant concrete implementations of DeterministicFunction (e.g. DF_Hyperbolic1.m) into the appropriate model folder.
• [x] this contains the model class, the JAGS and STAN models. This will require updating how the code knows the path of the JAGS and STAN models.

[drbenvincent]

Now we have easier folder structure. It's a bit easier to create new models and see what functions we need to write in order to do so. Resolving #152 will make it even easier to add new models.