Impute missing values in a feature set.

        Parameters
        ----------
        features: array-like {n_samples, n_features}
            A feature matrix

        Returns
        -------
        array-like {n_samples, n_features}

def _impute_values(self, features):
        """Impute missing values in a feature set.

        Parameters
        ----------
        features: array-like {n_samples, n_features}
            A feature matrix

        Returns
        -------
        array-like {n_samples, n_features}
        """
        if self.verbosity > 1:
            print('Imputing missing values in feature set')

        if self._fitted_imputer is None:
            self._fitted_imputer = Imputer(strategy="median")
            self._fitted_imputer.fit(features)

        return self._fitted_imputer.transform(features)