Hard to get working with SARSOP

[zsunberg]

I have backed us into a corner where it is hard to get QuickPOMDPs to work with SARSOP. This is because the POMDPX file format only supports (a, sp)-conditioned observation functions, but it also uses POMDPModelTools.StateActionReward, which requires the standard (s, a, sp) observation function.

If the user declares the observation function as an anonymous function inside of a QuickPOMDP constructor, there is no easy way to declare both methods. I am not sure what to do about this.

Current workaround is to declare the observation function outside of the constructor and define both methods.

[zsunberg]

Maybe we should use static_hasmethod from Tricks.jl? That seems sketchy though.