document Data::EventStream::Statistics::Sample
rename aggregator back

choosing the right name is difficult...
test for process aggregator
changes to window updates for count aggregators

- update window when time is set
- when event leaving window, move start_time to the time of the next event
- when adding first event move start_time to the time of this event
add Aggregator::Process module
fix event_stream_mixed for the new window updates
add get_iterator method to Window