Details for a specific user.
        Will pull from cached users first, or get and add to cached users.
        :param username: the username or userId of the user
        :return: VoicebaseUser

def get_user(self, user_id):
        """
        Details for a specific user.
        Will pull from cached users first, or get and add to cached users.
        :param username: the username or userId of the user
        :return: VoicebaseUser
        """
        if user_id in self._users:
            return self._users.get(user_id)
        else:
            # Load user
            # Save user in cache
            return