Write to outfiles.

def prt_outfiles_flat(self, goea_results, outfiles):
        """Write to outfiles."""
        kws = {'indent':self.args.indent, 'itemid2name':self.itemid2name}
        for outfile in outfiles:
            if outfile.endswith(".xlsx"):
                self.objgoea.wr_xlsx(outfile, goea_results, **kws)
            #elif outfile.endswith(".txt"):  # TBD
            #    pass
            else:
                self.objgoea.wr_tsv(outfile, goea_results, **kws)