property decorated method to get mean values of observation noise.
        This is a zero-valued pandas.Series

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

def mean_values(self):
        """ property decorated method to get mean values of observation noise.
        This is a zero-valued pandas.Series

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

        """
        vals = self.pst.observation_data.obsval.copy()
        vals.loc[self.names] = 0.0
        return vals