Write a python function union(set1, set2) that takes as input two sets of numbers, set1 and set2 and RETURNS a set of unique numbers appearing in EITHER set.

NOTE: we did this before with lists, but sets make this much easier.

>>> union({3, 4, 5, 8, 9, 10, 12}, {5, 6, 7}) \

... == {3, 4, 5, 6, 7, 8, 9, 10, 12}

True