docs: def set_cellpydata(self, cellpydata, cycle):
          """Performing fit of the OCV steps in the cycles set by set_cycles()
          from the data set by set_data()
  
          r is found by calculating v0 / i_start --> err(r)= err(v0) + err(i_start).
          c is found from using tau / r --> err(c) = err(r) + err(tau)
  
          Args:
              cellpydata (CellpyData): data object from cellreader
              cycle (int): cycle number to get from CellpyData object
  
          Returns:
              None
  
          """
          self.data = cellpydata
          self.step_table = self.data.dataset  # hope it works...
          time_voltage = self.data.get_ocv(direction='up',
                                           cycles=cycle)
          time = time_voltage.Step_Time
          voltage = time_voltage.Voltage
  
          self.time = np.array(time)
          self.voltage = np.array(voltage)