Redesign thread

[murrellb]

Now that we have a better idea of the space of possibilities here, there are some bits of this that could be tidied up a bit. Some of the changes will be within ForwardBackward, but we can track them all here.

Things to aim for:

• A distinction between the "Flow" process (which possibly includes a schedule, a restriction on a choice of losses, custom stepping procedures) and the underlying processes in ForwardBackward. Basically, all Flow processes should carry a little more information that helps the downstream stuff know what to do with them, and gives the user a more consistent experience.
• States need to have the notion of "an element" which might not be obvious from the state itself. Eg. if your tensor stores angles, you might want one element to be "three angles".