Allow overpayment on credit note refund and ability to allocate surplus to other credit notes

Just as with invoices where a payment can be made that is more than the invoice amount (the overpayment can then be used to allocate credit for other invoices), it should be possible to do the same with credit notes where a refund is made that is more than the credit note amount so that the surplus can be allocated to other credit notes.

The use case is that a single payment can be made for multiple credit notes.