Changes

gaming-contest: fixed various border cases in CombinatoricsHelper

This includes "invalid" cases like n<k in n-over-k as well as an one-off
error in returning true/false in CombinationSplitIterator.tryAdvance()
(according to the documentation, we have to return false if there is no
element *on entry* to process; we formerly returned false if there was
no element *on exit*). The unit tests were extended accordingly.