Registers a data type to encode/decode for xml settings.
        
        :param      typ     | <object>
                    encoder | <method>
                    decoder | <method>

def registerXmlType(typ, encoder, decoder):
        """
        Registers a data type to encode/decode for xml settings.
        
        :param      typ     | <object>
                    encoder | <method>
                    decoder | <method>
        """
        DataSet._xmlTypes[nstr(typ)] = (encoder, decoder)