docs: def load_mod_file(filename):
      """Load a .mod file (sometimes also called volt.dat or data.crt). This file
      contains the number of measurements in the first line, and in the following
      lines measurements. Each line consists of 4 columns:
  
      * the first column contains the current injection electrodes a and b,
        stored as one integer using the equation: a * 1e4 + b
      * the second column contains the voltage measurement electrodes m and n,
        stored as one integer using the equation: m * 1e4 + n
      * the third column contains the measured resistance [Ohm]
      * the fourth column contains the measured phase value [mrad]
  
      Parameters
      ----------
      filename : string
          Path of filename to import
  
      Returns
      -------
      df : pandas.DataFrame
          A reda-conform DataFrame
  
      Examples
      --------
  
          import reda
          import reda.importers.crtomo as cexp
          df = cexp.load_mod_file('volt_01_0.1Hz.crt')
          ert = reda.ERT(data=df)
  
      """
      df_raw = pd.read_csv(
          filename, skiprows=1, delim_whitespace=True,
          names=['ab', 'mn', 'r', 'rpha']
      )
      df_raw['Zt'] = df_raw['r'] * np.exp(1j * df_raw['rpha'] / 1000.0)
      df_raw['a'] = np.floor(df_raw['ab'] / 1e4).astype(int)
      df_raw['b'] = (df_raw['ab'] % 1e4).astype(int)
      df_raw['m'] = np.floor(df_raw['mn'] / 1e4).astype(int)
      df_raw['n'] = (df_raw['mn'] % 1e4).astype(int)
  
      df = df_raw.drop(['ab', 'mn'], axis=1)
      return df