Return the dataset representation in BMDS .(d) file.

def as_dfile(self):
        """
        Return the dataset representation in BMDS .(d) file.
        """
        rows = ["Dose NumAnimals Response Stdev"]
        for i, v in enumerate(self.doses):
            if i >= self.num_dose_groups:
                continue
            rows.append(
                "%f %d %f %f" % (self.doses[i], self.ns[i], self.means[i], self.stdevs[i])
            )
        return "\n".join(rows)