Option for Setting Gauss-Seidel Value Iteration

Currently, the solver only supports Gauss-Siedel value iteration. We should have an optional flag to do value iteration without the Gauss-Siedel method if the user wishes. There are a few ways to do this. The two most straightforward ways:

1. Have two different solver types with their respective solve functions

2. Have a flag that looks something like `gauss_siedel::Bool` as a field in the current `ValueIterationSolver` type without adding another type or `solve()` function.

Can this be done more elegantly? @zsunberg if I recall you were dealing with something similar to this in MCTS, any input here?