Created InventoryItem class in rpki.inv.item
KitchenItem and KitchenItemMetadata are now subclasses of InventoryItem
InventoryModel can now be used with a view
 - UPCs in the inventory are top level items in the view
 - InventoryItems are child items of their corresponding UPC item
TODO: Change InventoryWidget to use InventoryModel
TODO: Update documentation in Inventory class