docs: def register_extension(self, ext_in, ext_out, force=False):
          """
          Add/register a file extension.
  
  
          Args:
              ext_in (str): Extension of input files.
              ext_out (str): Extension of corresponding output files.
              force (bool): If ``force`` is set to ``True``, simply overwrite existing extensions, otherwise do nothing.
                  If the ``logger`` is set, log a warning about the duplicate extension if ``force == False``.
  
  
          """
          if not force and (ext_in in self.__extensions.keys()):
              self.log_warning("Extension %s already exist, ignore redefinition." % ext_in)
              return
  
          self.__extensions[ext_in] = ext_out