modify Union to be SetOp so it can do both union and difference
modify Union to be SetOp so it can do both union and difference

w1kke/hyperopt,jaberg/hyperopt,466152112/hyperopt,dallascard/hyperopt,beni55/hyperopt,dudalev/hyperopt,nicholas-leonard/hyperopt,allansp84/hyperopt,mathkann/hyperopt,CVML/hyperopt