Added more initialization options to aggr_node module.

Now can specify the result name and type of the aggregate.

Create ETS table for synopsis by default.
Defined synopsis helper functions.

Includes the following.
  (1) get_partials/2
  (2) add_partial/3
  (3) remove_partial/3
Defined function to compute aggregate over all partial results.
Created convenience functions to send messages to subscribers.

Defined send_tuples/2 and send_diffs/2.
Added support for receiving tuples and diffs.

Computes a new aggregate for each type and notifies its subscribers.
Made the atom `undefined' a special return value.

When the aggregate function produces a tuple with an undefined value,
it is not sent to the subscribers of the plan node.