Parses a filter arg in the format:
        <colname>__<op>
        :returns: colname, op tuple

def _parse_filter_arg(self, arg):
        """
        Parses a filter arg in the format:
        <colname>__<op>
        :returns: colname, op tuple
        """
        statement = arg.rsplit('__', 1)
        if len(statement) == 1:
            return arg, None
        elif len(statement) == 2:
            return statement[0], statement[1]
        else:
            raise QueryException("Can't parse '{}'".format(arg))