Get string describing counts of placed/unplaced GO IDs and count of sections.

def get_summary_str(self, sec2d_nt):
        """Get string describing counts of placed/unplaced GO IDs and count of sections."""
        data = self.get_summary_data(sec2d_nt)
        return "{M} GO IDs placed into {N} sections; {U} unplaced GO IDs".format(
            N=len(data['sections']), M=len(data['grouped']), U=len(data['ungrouped']))