Returns filtered list of features in this registry

        :param str api: Return only features with this api name, or None to
                        return all features.
        :return: list of Feature objects

def get_features(self, api=None):
        """Returns filtered list of features in this registry

        :param str api: Return only features with this api name, or None to
                        return all features.
        :return: list of Feature objects
        """
        return [x for x in self.features.values()
                if api and x.api == api or not api]