* Added MeterGroupID named tuple. #309
* modified functions to handle MeterGroupID:
  - __getitem__
  - from_list()
  - get_metergroupid()
* Also set default sample_period for dataframe_of_meters
* End result is the code now can do this sequence:
  groupby('type').select_top_k().plot(kind='area')
Handle case where old cache has different columns to new stats.  In
this case we remove the old stat.