Delete all features, which were not relevant in the fit phase.

        :param X: data sample with all features, which will be reduced to only those that are relevant
        :type X: pandas.DataSeries or numpy.array

        :return: same data sample as X, but with only the relevant features
        :rtype: pandas.DataFrame or numpy.array

def transform(self, X):
        """
        Delete all features, which were not relevant in the fit phase.

        :param X: data sample with all features, which will be reduced to only those that are relevant
        :type X: pandas.DataSeries or numpy.array

        :return: same data sample as X, but with only the relevant features
        :rtype: pandas.DataFrame or numpy.array
        """
        if self.relevant_features is None:
            raise RuntimeError("You have to call fit before.")

        if isinstance(X, pd.DataFrame):
            return X.copy().loc[:, self.relevant_features]
        else:
            return X[:, self.relevant_features]