This rather large change adds a layer between the underlying (immutable) model and the application.
In doing so we can avoid the use of a global state (useful for the purposes of configuring multiple models in the same application later on) and it also unlocks the ability to implement an MVC-like separation of concerns - again, the intention is that when it comes to comparisons, we will just be able to re-use our application views.
I was hoping that ``cara.state`` could have been avoided in lieu of using traitlets, but unfortunately I found a number of limitations which were prohibitive for its use here.
Foremost of which was the lack of first-class dataclass support and the difficulty in needing either to use instances of the model (immutable) or duplicate the model and its structure in a mutable form and use the ``traitlets.Instance`` type.
Instead I opted for doing it myself - the ``cara.state`` module would make a very good standalone project in the future.
