def register_extension(self, ext_in, ext_out, force=False):
if not force and (ext_in in self.__extensions.keys()):<EOL><INDENT>self.log_warning("<STR_LIT>" % ext_in)<EOL>return<EOL><DEDENT>self.__extensions[ext_in] = ext_out<EOL>
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``.