code: def add_sub_directory(self, key, path):
          
          sub_dir_path = os.path.join(self.results_root, path)
          os.makedirs(sub_dir_path, exist_ok=True)
          self._directories[key] = sub_dir_path
          return sub_dir_path
docs: Adds a sub-directory to the results directory.
  
          Parameters
          ----------
          key: str
              A look-up key for the directory path.
          path: str
              The relative path from the root of the results directory to the sub-directory.
  
          Returns
          -------
          str:
              The absolute path to the sub-directory.