Use ballot sync before threads are filtered out

Uses ballot sync before any if statements that filter out particular threads. As was necessary for !93 (merged)

Tests

Test runs /fred/oz016/tdavies/projects/testing/run_tdavies__use_ballot_sync_for_warp_reduce

test_zerolags_29_11_ballot_sync_run1_run2.txt
test_zerolags_29_11_tiebreak_ballot_sync.txt