Changes

* '''collectionView(_:layout:referenceSizeForFooterInSection:)''' - Required to return the size for the footer supplementary view for specified collection view section. Note that if a size is not specified, the view will not appear.

As previously discussed, the code for a typical application using a collection view will not directly create instances of either the cell or supplementary view classes. The reasoning behind this becomes evident when performance and memory requirements are taken into consideration. Consider, for example, a collection view that is required to display 1000 photo images. It can be assumed with a reasonable degree of certainty that only a small percentage of cells will be visible to the user at any one time. If the application were permitted to create each of the 1000 cells in advance the device would very quickly run into memory limitations.