docs: def _get_gtf_column(column_name, gtf_path, df):
      """
      Helper function which returns a dictionary column or raises an ValueError
      abou the absence of that column in a GTF file.
      """
      if column_name in df.columns:
          return list(df[column_name])
  
      else:
          raise ValueError(
              "Missing '%s' in columns of %s, available: %s" % (
                  column_name,
                  gtf_path,
                  list(df.columns)))