overload of Ensemble.copy()

        Returns
        -------
        ParameterEnsemble : ParameterEnsemble

def copy(self):
        """ overload of Ensemble.copy()

        Returns
        -------
        ParameterEnsemble : ParameterEnsemble

        """
        df = super(Ensemble,self).copy()
        pe = ParameterEnsemble.from_dataframe(df=df,pst=self.pst.get())
        pe.__istransformed = self.istransformed
        return pe