Add id property to book set to nil
make default status available set title and author as optional parameters.
built check out structure
Add validation for checking out books add books.check_in
set up empty array for books in stock
add add_book method and fix id property of Book.
changed add_book to work with book ids
know who borrow books and prevent double checkouts.
add cross functionality in check in and out methods across classes
set up validation for two books at a time rule, and correct test to reflect.
finished all tests