Prototype a stateful model-builder (with associated state widgets)

In order to allow comparison of model configurations, it is worth having a mechanism that allows us to hold on to (and mutate) the state of each of the model's inputs. The model itself is immutable (for performance and convenience reasons), so we will want to rebuild a model once a component of state changes. It would be great if we could expose this as a traitlet for which we can observe changes to the model in our application. In this way all of the application specific widgets will be defined in the python code rather than in the notebook.