Return a list of allowed values.

def allowed_values(self):
        """Return a list of allowed values."""
        if self._allowed_values is None:
            self._allowed_values = ValueList()
            for val in self.scraper._fetch_allowed_values(self):
                if isinstance(val, DimensionValue):
                    self._allowed_values.append(val)
                else:
                    self._allowed_values.append(DimensionValue(val,
                                                               Dimension()))
        return self._allowed_values