def add_log(self, group_name, field_name, log_string):
        """ Add a log the file.
        :param group_name: Global group name. Cave: No 'Analyses' is prepended.
        :param field_name: A field {group_name}/{field_name} will be written.
        :param log_string: content to be written.
        """
        self.assert_writeable()
        if group_name not in self.handle:
            self._add_group(group_name, {})
        sanitized_data = _sanitize_data_for_writing(log_string)
        self.handle[group_name].create_dataset(field_name, data=sanitized_data)
Add a log the file.
        :param group_name: Global group name. Cave: No 'Analyses' is prepended.
        :param field_name: A field {group_name}/{field_name} will be written.
        :param log_string: content to be written.