code: function(tdId) {
              let result = query('select * from questionnaire where td_id=?', [tdId]);
              if(result.length > 0) {
                  let questionnaire = result[0];
                  questionnaire.language_data = {};
  
                  // load data fields
                  let dataResults = query('select field, question_td_id from questionnaire_data_field where questionnaire_id=?',
                      [questionnaire.id]);
                  for(let data of dataResults) {
                      questionnaire.language_data[data.field] = data.question_td_id;
                  }
                  return questionnaire;
              }
              return null;
          }
docs: Returns a single questionnaire
  @param tdId {string} the translation database id of the questionnaire