get the omitted prior parameter covariance matrix

        Returns
        -------
        omitted_parcov : pyemu.Cov

        Note
        ----
        returns a reference
        
        If ErrorVariance.__omitted_parcov is None,
        attribute is dynamically loaded

def omitted_parcov(self):
        """get the omitted prior parameter covariance matrix

        Returns
        -------
        omitted_parcov : pyemu.Cov

        Note
        ----
        returns a reference
        
        If ErrorVariance.__omitted_parcov is None,
        attribute is dynamically loaded
        
        """
        if self.__omitted_parcov is None:
            self.log("loading omitted_parcov")
            self.__load_omitted_parcov()
            self.log("loading omitted_parcov")
        return self.__omitted_parcov