I added a test case for the YtY calculation code
I removed the indexes.quickSort() in the YtY calculation because I don't think it's necessary and the test cases passed without it. The order shouldn't matter since you're adding the scalers together. Correct me if i'm wrong.
In Factorization.java I added methods to access the iterator of item ids and user ids directly. This saves memory when using classes like TopItems.java when you don't have the DataModel class in memory as well.