forcing aggregateAndSample to select exactly nTotalEvents

[phauchamps]

in aggregateAndSample(), if nTotalEvents is greater than (number of flow frames) * (minimum frame #events), the actual number of events selected is less that `nTotalEvents. In some cases the actual number might actually be much lower.

Therefore it would be good to have a forceNTotalEvents = TRUE parameter mode, in which the nb of events selected per flow frame is not always balanced, to always obtain nTotalEvents (except when the total nb of available events, taking all flowFrames is less than nTotalEvents).