Abinit adds the idir-ipert index at the end of the 1DEN file and this breaks the extension
        e.g. out_DEN1. This method scans the files in the directories and returns a list of namedtuple
        Each named tuple gives the `path` of the 1DEN file and the `pertcase` index.

def find_1den_files(self):
        """
        Abinit adds the idir-ipert index at the end of the 1DEN file and this breaks the extension
        e.g. out_DEN1. This method scans the files in the directories and returns a list of namedtuple
        Each named tuple gives the `path` of the 1DEN file and the `pertcase` index.
        """
        regex = re.compile(r"out_DEN(\d+)(\.nc)?$")
        den_paths = [f for f in self.list_filepaths() if regex.match(os.path.basename(f))]
        if not den_paths: return None

        # Build list of (pertcase, path) tuples.
        pertfile_list = []
        for path in den_paths:
            name = os.path.basename(path)
            match = regex.match(name)
            pertcase, ncext = match.groups()
            pertfile_list.append((int(pertcase), path))

        # DSU sort.
        pertfile_list = sorted(pertfile_list, key=lambda t: t[0])
        return [dict2namedtuple(pertcase=item[0], path=item[1]) for item in pertfile_list]