docs: def get_dcap(self, cycle=None, dataset_number=None):
          """Returns discharge_capacity (in mAh/g), and voltage."""
  
          #  TODO: should return a DataFrame as default
          #  but remark that we then have to update e.g. batch_helpers.py
  
          dataset_number = self._validate_dataset_number(dataset_number)
          if dataset_number is None:
              self._report_empty_dataset()
              return
          dc, v = self._get_cap(cycle, dataset_number, "discharge")
          return dc, v