[Enhancement] Reset in Agent

[Jonathan2021]

Agent base class should probably have a reset method that would be called when the env is reset. This would allow to re-initialize agent variables each time a game is started. Reset in the env would call reset for the learning agent and opponent.

[glmcdona]

Added a new Agent callback on start of each game. This is automatically called on env.reset().

Callback added that you can override:

def game_start(self, game):
        """
        This funciton is called at the start of each game. Use this to
        reset and initialize per game. Note that self.team may have
        been changed since last game. The game map has been created
        and starting units placed.

        Args:
            game ([type]): Game.
        """
        pass