Return print format for Grouper, which includes hdr1usr01 and num_usrgos.

def _init_prtfmt(self, key="fmta"):
        """Return print format for Grouper, which includes hdr1usr01 and num_usrgos."""
        prtfmt = self.gosubdag.prt_attr[key]
        return prtfmt.replace("{NS}", "{NS} {hdr1usr01:2} {num_usrgos:>4} uGOs")