add. AggregatorMeta type.  This, combined with changes in numpy_op, allow greater flexibility in building aggregator methods from numpy_op.
add. AggregatorMeta type.  This, combined with changes in numpy_op, allow greater flexibility in building aggregator methods from numpy_op.

rfaulkner/wikipedia_user_metrics,wikimedia/user_metrics,wikimedia/user_metrics,rfaulkner/wikipedia_user_metrics,rfaulkner/wikipedia_user_metrics,wikimedia/analytics-user-metrics,wikimedia/analytics-user-metrics,wikimedia/analytics-user-metrics,wikimedia/user_metrics