* prompt_input() function to reduce code repetition
* initialisation function for DVD, CD, Game
* getters and setters for DVD and Game certs
* members current items is now [] rather than None
* member and item IDs are generated by the library class to prevent
clashes
* handle member/item not found in viewMember() and viewItem()
* started to make rentItem() and returnItem() in Library class and
checkInOut() function