intantiates the processing class (Direct or Grouped) and returns it.

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)