Read annotation file and store a list of namedtuples.

def _init_associations(self, fin_gpad, hdr_only=False):
        """Read annotation file and store a list of namedtuples."""
        ini = InitAssc(fin_gpad, self.godag)
        nts = ini.init_associations(hdr_only)
        self.hdr = ini.hdr
        return nts