docs: def import_sip04_data(data_filename):
      """Import RELEVANT data from the result files. Refer to the function
      :func:`reda.importers.sip04.import_sip04_data_all` for an importer that
      imports ALL data.
  
      Exported parameters:
  
      ================== ========================================================
      key                description
      ================== ========================================================
      a                  First current electrode
      b                  Second current electrode
      m                  First potential electrode
      n                  Second potential electrode
      frequency          Measurement frequency
      Temp_1             Temperature sensor 1 (optional)
      Temp_2             Temperature sensor 2 (optional)
      zt                 Complex Transfer Impedance (the measurement), mean value
      r                  Magnitude of mean measurements (=|zt|)
      rpha               Resistance phase [mrad]
      zt_1               Complex Transfer Impedance, first repetition
      zt_2               Complex Transfer Impedance, second repetition
      zt_3               Complex Transfer Impedance, third repetition
      ContactResistance  Contact resistance (mean value)
      ShuntResistance    Shunt resistance used [Ohm]
      ================== ========================================================
  
      Parameters
      ----------
      data_filename : string
          Path to .mat or .csv file containing SIP-04 measurement results. Note
          that the .csv file does not contain all data contained in the .mat
          file!
  
      Returns
      -------
      df : :py:class:`pandas.DataFrame`
          The data, contained in a DataFrame
  
      """
      df_all = import_sip04_data_all(data_filename)
      columns_to_keep = [
          'a', 'b', 'm', 'n',
          'frequency',
          'Temp_1', 'Temp_2',
          'Zm_1', 'Zm_2', 'Zm_3',
          'Zg_m',
          'zt',
          'Rs',
          'r',
          'rpha',
      ]
      df = df_all[columns_to_keep]
      df = df.rename(columns={
          'Rs': 'ShuntResistance',
          'Zg_m': 'ContactResistance',
          'Zm_1': 'zt_1',
          'Zm_2': 'zt_2',
          'Zm_3': 'zt_3',
      })
  
      return df