docs: def simple_db_engine(reader=None, srnos=None):
      """engine that gets values from the simple excel 'db'"""
  
      if reader is None:
          reader = dbreader.Reader()
          logger.debug("No reader provided. Creating one myself.")
  
      info_dict = dict()
      info_dict["filenames"] = [reader.get_cell_name(srno) for srno in srnos]
      info_dict["masses"] = [reader.get_mass(srno) for srno in srnos]
      info_dict["total_masses"] = [reader.get_total_mass(srno) for srno in srnos]
      info_dict["loadings"] = [reader.get_loading(srno) for srno in srnos]
      info_dict["fixed"] = [reader.inspect_hd5f_fixed(srno) for srno in srnos]
      info_dict["labels"] = [reader.get_label(srno) for srno in srnos]
      info_dict["cell_type"] = [reader.get_cell_type(srno) for srno in srnos]
      info_dict["raw_file_names"] = []
      info_dict["cellpy_file_names"] = []
  
      logger.debug("created info-dict")
  
      for key in list(info_dict.keys()):
          logger.debug("%s: %s" % (key, str(info_dict[key])))
  
      _groups = [reader.get_group(srno) for srno in srnos]
      logger.debug(">\ngroups: %s" % str(_groups))
      groups = helper.fix_groups(_groups)
      info_dict["groups"] = groups
  
      my_timer_start = time.time()
      filename_cache = []
      info_dict = helper.find_files(info_dict, filename_cache)
      my_timer_end = time.time()
      if (my_timer_end - my_timer_start) > 5.0:
          logger.info(
              "The function _find_files was very slow. "
              "Save your info_df so you don't have to run it again!"
          )
  
      info_df = pd.DataFrame(info_dict)
      info_df = info_df.sort_values(["groups", "filenames"])
      info_df = helper.make_unique_groups(info_df)
  
      info_df["labels"] = info_df["filenames"].apply(helper.create_labels)
      info_df.set_index("filenames", inplace=True)
      return info_df