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('[FeedbackResults]._upgrade()')
        if hasattr(self, 'version'):
            version = Version.fromstring(self.version)
        else:
            version = Version(0)
        logging.debug('[FeedbackResults] version=%s, class_version=%s' %
                      (str(version), self.class_version))
        if version > Version.fromstring(self.class_version):
            logging.debug('[FeedbackResults] version>class_version')
            raise FutureVersionError(Version.fromstring(self.class_version),
                                     version)
        elif version < Version.fromstring(self.class_version):
            if version < Version(0, 1):
                self.calibration = FeedbackCalibration()
            if version < Version(0, 2):
                # flag invalid data points
                self.version = str(Version(0, 2))
                self.fb_resistor[self.V_fb > 5] = -1
                self.hv_resistor[self.V_hv > 5] = -1
            if version < Version(0, 3):
                self.attempt = 0
            if version < Version(0, 4):
                del self.sampling_time_ms
                del self.delay_between_samples_ms
                self.voltage = self.options.voltage
                del self.options
                del self.attempt
            if version < Version(0, 5):
                self.area = 0
                self.version = str(Version(0, 5))
            if version < Version(0, 6):
                self.amplifier_gain = None
                self.vgnd_hv = None
                self.vgnd_fb = None
                self.version = str(Version(0, 6))
                logging.info('[FeedbackResults] upgrade to version %s' %
                             self.version)
        else:
            # Else the versions are equal and don't need to be upgraded.
            pass