Reducers aren't implemented

[nickrsan]

Reducers have a skeleton, but aren't actually implemented yet and have no point that they are automatically called if provided.

Every other spot in the code that utilizes state variables does take into account multiple state variables, and in fact, you can still run a multi-state problem, so long as all states represent a choice rather than an external uncertainty, because each state can use an availability_function to dictate which options remain at each stage.

When some states are externally driven and uncertain (eg, stochastic dynamic programs), then a reducer may be necessary, so it would be good to implement them.