the mean value vector while respecting log transform

        Returns
        -------
        mean_values : pandas.Series

def mean_values(self):
        """ the mean value vector while respecting log transform

        Returns
        -------
        mean_values : pandas.Series

        """
        if not self.istransformed:
            return self.pst.parameter_data.parval1.copy()
        else:
            # vals = (self.pst.parameter_data.parval1 *
            #         self.pst.parameter_data.scale) +\
            #         self.pst.parameter_data.offset
            vals = self.pst.parameter_data.parval1.copy()
            vals[self.log_indexer] = np.log10(vals[self.log_indexer])
            return vals