code: def _clean_header_df(self, df):
          
          if self.suffix == '-drvd.txt':
              df.units = {'release_time': 'second',
                          'precipitable_water': 'millimeter',
                          'inv_pressure': 'hPa',
                          'inv_height': 'meter',
                          'inv_strength': 'Kelvin',
                          'mixed_layer_pressure': 'hPa',
                          'mixed_layer_height': 'meter',
                          'freezing_point_pressure': 'hPa',
                          'freezing_point_height': 'meter',
                          'lcl_pressure': 'hPa',
                          'lcl_height': 'meter',
                          'lfc_pressure': 'hPa',
                          'lfc_height': 'meter',
                          'lnb_pressure': 'hPa',
                          'lnb_height': 'meter',
                          'lifted_index': 'degC',
                          'showalter_index': 'degC',
                          'k_index': 'degC',
                          'total_totals_index': 'degC',
                          'cape': 'Joule / kilogram',
                          'convective_inhibition': 'Joule / kilogram'}
  
          else:
              df.units = {'release_time': 'second',
                          'latitude': 'degrees',
                          'longitude': 'degrees'}
  
          return df
docs: Format the header dataframe and add units.