docs: def return_multiple_convert_numpy(self, start_id, end_id, converter, add_args=None):
          """
          Converts several objects, with ids in the range (start_id, end_id)
          into a 2d numpy array and returns the array, the conversion is done by the 'converter' function
  
          Parameters
          ----------
          start_id : the id of the first object to be converted
          end_id : the id of the last object to be converted, if equal to -1, will convert all data points in range
          (start_id, <id of last element in database>)
          converter : function, which takes the path of a data point and *args as parameters and returns a numpy array
          add_args : optional arguments for the converter (list/dictionary/tuple/whatever). if None, the
          converter should take only one input argument - the file path. default value: None
  
          Returns
          -------
          result : 2-dimensional ndarray
          """
          if end_id == -1:
              end_id = self.points_amt
          return return_multiple_convert_numpy_base(self.dbpath, self.path_to_set, self._set_object, start_id, end_id,
                                                    converter, add_args)