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

def as_dfile(self):
        """
        Return the dataset representation in BMDS .(d) file.
        """
        rows = ["Dose Response"]
        for dose, response in zip(self.individual_doses, self.responses):
            dose_idx = self.doses.index(dose)
            if dose_idx >= self.num_dose_groups:
                continue
            rows.append("%f %f" % (dose, response))
        return "\n".join(rows)