Get or create a new case specific gene list record.

def case_genelist(self, case_obj):
        """Get or create a new case specific gene list record."""
        list_id = "{}-HPO".format(case_obj.case_id)
        gene_list = self.gene_list(list_id)

        if gene_list is None:
            gene_list = GeneList(list_id=list_id)
            case_obj.gene_lists.append(gene_list)
            self.session.add(gene_list)

        return gene_list