Returns the highest whole-number unit.

def auto(self):
        """Returns the highest whole-number unit."""
        if self._value >= 1099511627776:
            return self.TiB, 'TiB'
        if self._value >= 1073741824:
            return self.GiB, 'GiB'
        if self._value >= 1048576:
            return self.MiB, 'MiB'
        if self._value >= 1024:
            return self.KiB, 'KiB'
        else:
            return self.B, 'B'