docs: def dqdv_cycle(cycle, splitter=True, **kwargs):
      """Convenience functions for creating dq-dv data from given capacity and
      voltage cycle.
  
      Returns the a DataFrame with a 'voltage' and a 'incremental_capacity'
      column.
  
          Args:
              cycle (pandas.DataFrame): the cycle data ('voltage', 'capacity',
                   'direction' (1 or -1)).
              splitter (bool): insert a np.NaN row between charge and discharge.
  
          Returns:
              List of step numbers corresponding to the selected steptype.
                  Returns a pandas.DataFrame
              instead of a list if pdtype is set to True.
  
          Example:
              >>> cycle_df = my_data.get_cap(
              >>> ...   1,
              >>> ...   categorical_column=True,
              >>> ...   method = "forth-and-forth"
              >>> ... )
              >>> voltage, incremental = ica.dqdv_cycle(cycle_df)
  
      """
  
      c_first = cycle.loc[cycle["direction"] == -1]
      c_last = cycle.loc[cycle["direction"] == 1]
  
      converter = Converter(**kwargs)
      converter.set_data(c_first["capacity"], c_first["voltage"])
      converter.inspect_data()
      converter.pre_process_data()
      converter.increment_data()
      converter.post_process_data()
      voltage_first = converter.voltage_processed
      incremental_capacity_first = converter.incremental_capacity
  
      if splitter:
          voltage_first = np.append(voltage_first, np.NaN)
          incremental_capacity_first = np.append(incremental_capacity_first,
                                                 np.NaN)
  
      converter = Converter(**kwargs)
      converter.set_data(c_last["capacity"], c_last["voltage"])
      converter.inspect_data()
      converter.pre_process_data()
      converter.increment_data()
      converter.post_process_data()
      voltage_last = converter.voltage_processed[::-1]
      incremental_capacity_last = converter.incremental_capacity[::-1]
      voltage = np.concatenate((voltage_first,
                                voltage_last))
      incremental_capacity = np.concatenate((incremental_capacity_first,
                                             incremental_capacity_last))
  
      return voltage, incremental_capacity