Write png containing plot of GoSubDag.

def wrplt(self, fout_dir, plt_ext="png"):
        """Write png containing plot of GoSubDag."""
        # Ex basename
        basename = self.grprobj.get_fout_base(self.ntplt.hdrgo)
        plt_pat = self.get_pltpat(plt_ext)
        fout_basename = plt_pat.format(BASE=basename)
        fout_plt = os.path.join(fout_dir, fout_basename)
        self.gosubdagplot.plt_dag(fout_plt) # Create Plot
        return fout_plt