Compute number of players, both human and computer.

def _num_players(self):
        """Compute number of players, both human and computer."""
        self._player_num = 0
        self._computer_num = 0
        for player in self._header.scenario.game_settings.player_info:
            if player.type == 'human':
                self._player_num += 1
            elif player.type == 'computer':
                self._computer_num += 1