get the deviations of the ensemble value from the mean vector

        Returns
        -------
            en : pyemu.Ensemble
                Ensemble of deviations from the mean

def get_deviations(self):
        """get the deviations of the ensemble value from the mean vector

        Returns
        -------
            en : pyemu.Ensemble
                Ensemble of deviations from the mean
        """

        mean_vec = self.mean()

        df = self.loc[:,:].copy()
        for col in df.columns:
            df.loc[:,col] -= mean_vec[col]
        return type(self).from_dataframe(pst=self.pst,df=df)