docs: def get_database_data(file_name=''):
      """return the energy (eV) and Sigma (barn) from the file_name
      
      Parameters:
      ===========
      file_name: string ('' by default) name of csv file
      
      Returns:
      ========
      pandas dataframe
      
      Raises:
      =======
      IOError if file does not exist
      """
      if not os.path.exists(file_name):
          raise IOError("File {} does not exist!".format(file_name))
      df = pd.read_csv(file_name, header=1)
      return df