eltype for stepthrough

[zsunberg]

Currently collect(stepthrough(...)) returns a Vector{Any}. It might be nice to return something more concrete. This involves implementing eltype(::(PO)MDPSimIterator).

This should be pretty straightforward for states, actions, observations, reward, and time. It is a bit more complex for beliefs and info. It might require something like belief_type(updater, pomdp), which would not be too hard to make.