The AI is not competitive

[chu1248]

https://github.com/zou3519/acquire/blob/ea1b0af7c09b31e554d68af7da85e92b37902507/src/mcts/ISTreeNode.scala#L51

The node scoring simply considers its own score in competitive game. It should probably consider the differences between itself and other opponents, e.g. stats(i) += result(i) * result.length - result.sum

(PITreeNode also experience the same problem.)