Implement Monte Carlo Tree Search

[PascalCremer]

One can implement MCTS based on the value model to improve model performance. This is not intended to be used during data generation due to the high cost of computation.

[cleeff]

I implemented a first MCTS version which can already me used in interactive mode. I thought about implementing this as a model wrapper but the interface doesn't quite match. Maybe we can find a way to merge the two interfaces s.t. MCTS can be used in comparison with models without MCTS without too much extra code.

c_puct Parameter certainly needs to be tuned. Also currently the prior policy is set to 1 uniformly. Softmax of the move evaluation might make more sense and should certainly be tried.

[cleeff]

I just added a sigmoid(model_output) factor to U. Otherwise the first move will be entirely random. Now it looks much better.