def extract_words data_set, keep_label=false
      data_set.map do |data|
        extract_words_from_data data, keep_label
      end
    end
def extract_words data_set, keep_label=false
      data_set.map do |data|
        extract_words_from_data data, keep_label
      end
    end
extracts the words of all provided data entries
 @param  data_set [Array<PreprocessedData>] list of preprocessed data
 @param  keep_label

 @return [Array<OpenStruct<Array<String>,Boolean>>] list of words per data entry