Vanilla MCTS (without networks) for testing

[gemasphi]

Hey,

First of all thanks for the implementation I've been finding it very useful for my own experiments.

I implemented vanilla MCTS using rollouts to evaluate a position, which can be used for testing MuZero models in games where we have access to a perfect simulation of the environment (Board games etc). It can be used through the command line, as I added an option for that. Or by changing the game config (self.opponent = "MCTS")

I tested it with Connect Four, but I would like you to test it as well.