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

        Example
        -------
        >>> print(dataset.as_dfile())
        Dose Incidence NEGATIVE_RESPONSE
        0.000000 5 70
        1.960000 1 48
        5.690000 3 47
        29.750000 14 35

def as_dfile(self):
        """
        Return the dataset representation in BMDS .(d) file.

        Example
        -------
        >>> print(dataset.as_dfile())
        Dose Incidence NEGATIVE_RESPONSE
        0.000000 5 70
        1.960000 1 48
        5.690000 3 47
        29.750000 14 35
        """
        rows = ["Dose Incidence NEGATIVE_RESPONSE"]
        for i, v in enumerate(self.doses):
            if i >= self.num_dose_groups:
                continue
            rows.append("%f %d %d" % (self.doses[i], self.incidences[i], self.remainings[i]))
        return "\n".join(rows)