Parameters
        ----------
        source : string
            name of the ReadSource

        allele_group : string
            one of: num_ref, num_alt, total_depth

        Returns
        ----------
        column name : string

def column_name(self, source, allele_group):
        """
        Parameters
        ----------
        source : string
            name of the ReadSource

        allele_group : string
            one of: num_ref, num_alt, total_depth

        Returns
        ----------
        column name : string
        """
        return self.column_format.format(
            source=source,
            allele_group=allele_group)