window_aggregate sdk example

[esoroush]

WindowAggregate sdk example:
A WindowAggregate operator written by gl-sdk that accepts a BASE_AGGREGATE as an argument and does sliding window aggregation.

Any aggregate operator that inherits from BASE_AGGREGATE should provide implementation for the following methods in order to be used by WindowAggregate function:

- initiate()
- add_element()
- remove_element() (optional: if incremental computation is supported)
- emit()

Currently MIN,MAX,SUM,COUNT and AVG are implemented based on this framework.

[haijieg]

if you use std::list or std::deque, you can avoid creating a new queue here.