reset the private Pst.res attribute

        Parameters
        ----------
        res : (varies)
            something to use as Pst.res attribute

def set_res(self,res):
        """ reset the private Pst.res attribute

        Parameters
        ----------
        res : (varies)
            something to use as Pst.res attribute

        """
        if isinstance(res,str):
            res = pst_utils.read_resfile(res)
        self.__res = res