class_docstr: This class defines a simple aggregator that operates on groups of entries 
  in a pandas dataframe
  
  `func` should accept a dataframe and return a list of dataframes with appropriate indices
class_docstr_tok: ["This","class","defines","a","simple","aggregator","that","operates","on","groups","of","entries","in","a","pandas","dataframe","`","func","`","should","accept","a","dataframe","and","return","a","list","of","dataframes","with","appropriate","indices"]
code: def __call__(self, result_dict):
      return self.func(result_dict[self.group_by])
  
imports: from __future__ import absolute_import, division, print_function
  import itertools
  from itertools import groupby
  from collections import defaultdict
  import pandas
  from niftynet.engine.application_factory import EvaluationFactory
  from niftynet.utilities.util_common import cache
label_desc: Use image node_id 2 for calling the DataFrameAggregator obj's underlying member method code with example usage: obj.__call__(result_dict) and returns: self