docs: def create_tmp_rootdir(self):
          """
          create a rootdir that we can destroy later again
  
          Returns
          -------
  
          """
          if self.rootdir:
              tmp_rootdir = tempfile.mkdtemp(prefix='bcolz-')
              self._dir_clean_list.append(tmp_rootdir)
          else:
              tmp_rootdir = None
          return tmp_rootdir