Upgrade the serialized object if necessary.

        Raises:
            FutureVersionError: file was written by a future version of the
                software.

def _upgrade(self):
        """
        Upgrade the serialized object if necessary.

        Raises:
            FutureVersionError: file was written by a future version of the
                software.
        """
        logging.debug("[FeedbackResultsSeries]._upgrade()")
        version = Version.fromstring(self.version)
        logging.debug('[FeedbackResultsSeries] version=%s, class_version=%s',
                      str(version), self.class_version)
        if version > Version.fromstring(self.class_version):
            logging.debug('[FeedbackResultsSeries] version>class_version')
            raise FutureVersionError(Version.fromstring(self.class_version),
                                     version)
        elif version < Version.fromstring(self.class_version):
            if version < Version(0, 1):
                self.time = [None]*len(self.data)
                self.version = str(Version(0, 1))