Append the result for one run. Needs to be called before collect_data().

def append(self, resultFile, resultElem, all_columns=False):
        """
        Append the result for one run. Needs to be called before collect_data().
        """
        self._xml_results += [(result, resultFile) for result in _get_run_tags_from_xml(resultElem)]
        for attrib, values in RunSetResult._extract_attributes_from_result(resultFile, resultElem).items():
            self.attributes[attrib].extend(values)

        if not self.columns:
            self.columns = RunSetResult._extract_existing_columns_from_result(resultFile, resultElem, all_columns)