overload of Ensemble.copy()

        Returns
        -------
        ObservationEnsemble : ObservationEnsemble

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

        Returns
        -------
        ObservationEnsemble : ObservationEnsemble

        """
        df = super(Ensemble,self).copy()
        return type(self).from_dataframe(df=df,pst=self.pst.get())