Request the current number of players for a given app.

        appID: The app ID
        format: Return format. None defaults to json. (json, xml, vdf)

def get_number_of_current_players(self, appID, format=None):
        """Request the current number of players for a given app.

        appID: The app ID
        format: Return format. None defaults to json. (json, xml, vdf)

        """
        parameters = {'appid' : appID}
        if format is not None:
            parameters['format'] = format
        url = self.create_request_url(self.interface,
            'GetNumberOfCurrentPlayers', 1, parameters)
        data = self.retrieve_request(url)
        return self.return_data(data, format=format)