Guarantee order in cross2gen_helper

[jeanluct]

After fixing #116, the C++ code now computes the correct braid for the taffy pullers. However, different runs can return generators in different orders, because of the parallelization. The braid is still the same, but it would be preferable for the code to be deterministic.

(If we fix this also update taffyTest to use stronger verifyEqual, which is commented-out.)