Returns the numerator with formatting.

def str_numerator(self):
        """Returns the numerator with formatting."""
        if not self.undefined:
            return None
        unit_numerator, unit = self._unit_class(self.numerator).auto
        formatter = '%d' if unit_numerator == self.numerator else '%0.2f'
        numerator = locale.format(formatter, unit_numerator, grouping=True)
        return '{0} {1}'.format(numerator, unit)