docs: 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]