[4pt] Implement methods

[HOUDA MZARI]

• add_item method in the Genre 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 MusicAlbum class

    should override the method from the parent class
    should return true if parent's method returns true AND if on_spotify equals true
    otherwise, it should return false