[2pt] (Team member #1) Implement methods

[abel-tefera]

• add_itemmethod in the Label class
  • should take an instance of the Item class as an input
  • should add the input item to the collection of items
  • should add self as a property of the item object (by using the correct setter from the item object)
• can_be_archived?() in the Book class
  • should override the method from the parent class
  • should return true if parent's method returns true OR if cover_state equals to "bad"
  • otherwise, it should return false