Ensures a subdirectory of the working directory.

        Parameters
        ----------
        path_parts : iterable[str]
            The parts of the path after the working directory.

def ensure_dir(self, *path_parts):
        """Ensures a subdirectory of the working directory.

        Parameters
        ----------
        path_parts : iterable[str]
            The parts of the path after the working directory.
        """
        path = self.getpath(*path_parts)
        ensure_directory(path)
        return path