mod/add - flesh out Aggregator class / Create AggregatorStatOp subclass.

Change-Id: I048c77d8b07df9b948598891e7228d1988cc8016
add - method _build_agg_items.

Change-Id: I00b710016bfbc8ab9e1e7ec967eb4103e7898b02
add - header to AggregatorStatOp.

Change-Id: I98baa01d920473961b7735e872390cd66182c1c1
add - run method for AggregatorStatOp.

Change-Id: I031af4aaa42cb075fb1024c038f1e0be77384877
add - aggregator_meta namedtuple to Aggregator class.

Change-Id: I4a4d71452ebcf65894e4055b15fedb0b1c0e13fd