def extend(self, features):
        """ Add a list of features to the bag.

        Parameters
        ----------
        feature : :obj:`list` of :obj:`Feature`
            features to add
        """
        self.features_.extend(features)
        self.num_features_ = len(self.features_)
Add a list of features to the bag.

        Parameters
        ----------
        feature : :obj:`list` of :obj:`Feature`
            features to add