Fix EventSelectorAthenaPool input file switching triggered by InputCollections property change - ATEAM-842

The modification of the EventSelectorAthenaPool InputCollections property triggers an immediate update callback that resets the EvSelector.
This MR delays the reset to the next execution of EventSelector::next() (the callback now just sets a notification flag).
Also, after resetting the EvSelector, the next() will not advance to the next event, because that would mean a jump over the first even in the new input. Which is most likely the reason for problems described in ATEAM-842.