Request a list of groups a user is subscribed to.

        steamID: User ID
        format: Return format. None defaults to json. (json, xml, vdf)

def get_user_group_list(self, steamID, format=None):
        """Request a list of groups a user is subscribed to.

        steamID: User ID
        format: Return format. None defaults to json. (json, xml, vdf)

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