Returns the highest whole-number unit.

def auto(self):
        """Returns the highest whole-number unit."""
        if self._value >= 1000000000000:
            return self.tb, 'tb'
        if self._value >= 1000000000:
            return self.gb, 'gb'
        if self._value >= 1000000:
            return self.mb, 'mb'
        if self._value >= 1000:
            return self.kb, 'kb'
        else:
            return self.b, 'b'