docs: def write_files_to_directory(df, directory, **kwargs):
      """Write sEIT data ta files. Data of each frequency is written in a
      separate file that conforms to the CRMod/CRTomo standard, and can directly
      be used for inversions using CRTomo.
  
      Parameters
      ----------
      df : pandas.DataFrame
          Data
      directory : string
          Output directory. Will be created if not existant
  
      Other Parameters
      ----------------
      store_errors: bool, optional
          store the device generated errors in the output files (as additional
          columns). Default: False
      norrec: string, optional
          all|nor|rec which normal-reciprocal data set to use. Default: all
  
      """
      if 'frequency' in df.columns:
          group_key = 'frequency'
      elif 'frequency_[Hz]' in df.columns:
          group_key = 'frequency_[Hz]'
      else:
          group_key = None
  
      if not os.path.isdir(directory):
          os.makedirs(directory)
  
      pwd = os.getcwd()
      os.chdir(directory)
  
      if group_key is not None:
          g = df.groupby(group_key)
  
          nr = 1
          frequencies_used = []
          for frequency, group in sorted(g):
              if group.shape[0] > 0:
                  frequencies_used.append(frequency)
              filename = 'volt_{0:02}_{1:.6}Hz.crt'.format(nr, frequency)
              save_block_to_crt(
                  filename,
                  group,
                  norrec=kwargs.get('norrec', 'all'),
                  store_errors=kwargs.get('store_errors', False),
              )
  
              nr += 1
          np.savetxt('frequencies.dat', frequencies_used)
      else:
          save_block_to_crt(
              'volt.dat',
              df,
              norrec=kwargs.get('norrec', 'all'),
              store_errors=kwargs.get('store_errors', False),
          )
  
      os.chdir(pwd)