You are to develop software for a simple online shopping situation. The entities in
this scenario include the web shop, customers, products, orders and invoices. The
shop has a list of products available. A customer logs in, selects products and adds
them to his/her order. When the shopping is complete an invoice with unique invoice
number is generated for the order. There are 10 products in this simple example and
they should be stored in an array. Products can be added and deleted from an order.
Only one customer at a time need be considered.