'Expand tree by creating new children.
Arguments:
action_priors -- output from policy function - a list of tuples of actions and their prior
probability according to the policy function.
Returns:
None'

def expand(self, action_priors):

    for (action, prob) in action_priors:
        if (action not in self._children):
            self._children[action] = TreeNode(self, prob)
