rma_account: Add flag in customer operations to default the refund as free of charge

[JordiBForgeFlow]

In the scenario where a company uses anglo-saxon accounting, if you receive products from a customer and don't expect to refund the customer but send a replacement unit you still need to create a debit entry on the Stock Interim (Delivered) account. In order to do this the best approach is to create a customer refund from the RMA, but set as free of charge (price unit = 0). The refund will be 0, but the Stock Interim (Delivered) account will be posted anyways.