Allow to edit customer on purchase request
For now the customer is only filled by the sale when creating the purchase request based on the product supplier information. But sometimes the purchaser may want to force a drop shipment for specific purchase request with sale as origin. In this case we could allow them to edit the customer field as long as it is the same as on the sale (shipment party or party), the delivery address should be filled by default with the shipment address but can still be edited for another address.