Includes Shipment as parameter of Generate Invoice Process

This patch allows to select a shipment as parameter of invoice generation. It's useful when you have a single shipment for multiple orders. The current behavior to achieve this result you'll need to use the Generate Invoices form and select the exactly same orders of shipments or you can use the process and filter by business partner, but the process will generate invoices for all shipments of this business partner. With this patch you'll be able to select an existing shipment.