IT FRESH-591

#292 Automatically add reference no from purchase order to invoice candidate

Testcase to check if the reference no is set in invoice candidates

Reference number propagation

1. Create a new purchase order for bpartner G000X and add 2 products, P0001, P0002

2. set a reference number, save and complete

3. zoom in on the created invoice candidates
   • => 2 invoice candidates are created, for products P0001 and P0002. Both have the reference number you set on step 2
4. Go to receipt pos and create 2 inouts (1 per product)
   • => 2 inouts are created and both have the reference number empty
5. zoom in to invoice candidates
   • => new invoice candidates are created for the handling units. All have the reference number you set on step 2
6. Go to gear and create an invoice
   • => invoice was created with the same reference number