This module extends the functionality of purchase orders to better
manage supplier returns and refunds.

In the purchase order you are able to display, for each line:

- Billed Quantity and Refunded Quantity, as separate fields.
- Received Quantity and Returned Refundable Quantity, as separate
  fields.

You have the option to create a vendor bill or a refund. In the bill or
refund the correct quantity will be proposed, based on those fields.
