docs: def get_group_by_class(self, definition):
          """
              intantiates the processing class (Direct or Grouped) and returns it.
          """
          group_by = definition["group"]
          series = definition["series"]
          if "formatter" in definition:
              formatter = {group_by: definition["formatter"]}
          else:
              formatter = {}
          return self.ProcessClass([group_by], series, formatter)