code: def on_all_ok(self):
          
          # Merge DDB files.
          out_ddb = self.merge_ddb_files()
          return self.Results(node=self, returncode=0, message="DDB merge done")
docs: This method is called when all tasks reach S_OK
          Ir runs `mrgddb` in sequential on the local machine to produce
          the final DDB file in the outdir of the `Work`.