ADD: list_average aggregator method for computing averages on a key in a list
ADD: list_average aggregator method for computing averages on a key in a list

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