code: def check_output_files(self,
                             return_found=True,
                             return_missing=True):
          
          all_output_files = self.files.chain_output_files + \
              self.sub_files.chain_output_files
          return check_files(all_output_files, self._file_stage,
                             return_found, return_missing)
docs: Check if output files exist.
  
          Parameters
          ----------
          return_found : list
              A list with the paths of the files that were found.
  
          return_missing : list
              A list with the paths of the files that were missing.
  
          Returns
          -------
          found : list
              List of the found files, if requested, otherwise `None`
  
          missing : list
              List of the missing files, if requested, otherwise `None`