get the pyemu.Pst attribute

        Returns
        -------
        pst : pyemu.Pst

        Note
        ----
        returns a references
        
        If LinearAnalysis.__pst is None, then the pst attribute is
        dynamically loaded before returning

def pst(self):
        """ get the pyemu.Pst attribute

        Returns
        -------
        pst : pyemu.Pst

        Note
        ----
        returns a references
        
        If LinearAnalysis.__pst is None, then the pst attribute is
        dynamically loaded before returning
        """
        if self.__pst is None and self.pst_arg is None:
            raise Exception("linear_analysis.pst: can't access self.pst:" +
                            "no pest control argument passed")
        elif self.__pst:
            return self.__pst
        else:
            self.__load_pst()
            return self.__pst