Given a header GO, return the sections that contain it.

def get_sections(self, hdrgo, dflt_section=True):
        """Given a header GO, return the sections that contain it."""
        dflt_list = []
        # If the hdrgo is not in a section, return the default name for a section
        if dflt_section:
            dflt_list = [self.secdflt]
        return self.hdrgo2sections.get(hdrgo, dflt_list)