docs: def export_bert(data, electrodes, filename):
      """Export to unified data format used in pyGIMLi & BERT.
  
      Parameters
      ----------
      data : :py:class:`pandas.DataFrame`
          DataFrame with at least a, b, m, n and r.
      electrodes : :py:class:`pandas.DataFrame`
          DataFrame with electrode positions.
      filename : str
          String of the output filename.
      """
      # Check for multiple timesteps
      if has_multiple_timesteps(data):
          for i, timestep in enumerate(split_timesteps(data)):
              export_bert(timestep, electrodes,
                          filename.replace(".", "_%.3d." % i))
  
          # TODO: Make ABMN consistent
          # index_full = ert.data.groupby(list("abmn")).groups.keys()
          # g = ert.data.groupby('timestep')
          # q = ert.data.pivot_table(values='r', index=list("abmn"), columns="timestep", dropna=True)
          # ert.data.reset_index(list("abmn"))
  
      f = open(filename, 'w')
      f.write("%d\n" % len(electrodes))
      f.write("# ")
  
      # Make temporary copies for renaming
      electrodes = electrodes.copy()
      data = data.copy()
  
      electrodes.columns = electrodes.columns.str.lower()
      data.columns = data.columns.str.lower()
  
      # Remove unnecessary columns and rename according to bert conventions
      # https://gitlab.com/resistivity-net/bert#the-unified-data-format
      cols_to_export = ["a", "b", "m", "n", "u", "i", "r", "rho_a", "error"]
      data.drop(data.columns.difference(cols_to_export), 1, inplace=True)
      data.rename(columns={"rho_a": "rhoa", "error": "err"}, inplace=True)
  
      for key in electrodes.keys():
          f.write("%s " % key)
      f.write("\n")
      for row in electrodes.itertuples(index=False):
          for val in row:
              f.write("%5.3f " % val)
          f.write("\n")
      f.write("%d\n" % len(data))
      f.write("# ")
  
      # Make sure that a, b, m, n are the first 4 columns
      columns = data.columns.tolist()
      for c in "abmn":
          columns.remove(c)
      columns = list("abmn") + columns
      data = data[columns]
  
      for key in data.keys():
          f.write("%s " % key)
      f.write("\n")
      for row in data.itertuples(index=False):
          for i, val in enumerate(row):
              if i < 4:
                  f.write("%d " % val)
              else:
                  f.write("%E " % val)
  
          f.write("\n")
      f.close()