Simplify model state representation

[unixpickle]

Right now, there's three different types a model state can be: NoneType, an array, or a tuple of arrays. In the future, it would be nice to unify this.

I propose that states will always be tuples. An empty tuple corresponds to a stateless model; otherwise, the tuple may contain one element (the standard state case), or more than one element. Also, instead of a stateful property, there could just be a num_states property that indicates how many tuple elements are in the state.

[unixpickle]

Note that this will be a huge breaking change. It won't be happening until 0.13.0.