Create dict with taxid keys and annotation namedtuple list.

def _init_taxid2asscs(self):
        """Create dict with taxid keys and annotation namedtuple list."""
        taxid2asscs = cx.defaultdict(list)
        for ntanno in self.associations:
            taxid2asscs[ntanno.tax_id].append(ntanno)
        assert len(taxid2asscs) != 0, "**FATAL: NO TAXIDS: {F}".format(F=self.filename)
        # """Print the number of taxids stored."""
        prt = sys.stdout
        num_taxids = len(taxid2asscs)
        prt.write('{N} taxids stored'.format(N=num_taxids))
        if num_taxids < 5:
            prt.write(': {Ts}'.format(Ts=' '.join(sorted(str(t) for t in taxid2asscs))))
        prt.write('\n')
        return dict(taxid2asscs)