no population of invoices with a cash discount payment term in time [account_payment_discount_extension]

[Thorsten Vocks (OpenBig.org)]

The population seems to check the due date and the result is good for invoices with a payment term
without cash discounts. Invoices with cash discount lines are not populated into payment lines in time.
In result i do not receive a proposal for a payment neither to early nor to late, in fact i only receive a
proposal also when the invoice is due without deduction of cash discount.

My use case was:

* Invoice with payment term like "10 days with 2% - 30 days 100% Net" (only 1 line in table cash discount conditions)
* On opening of new invoice the invoice date is "today".
* Last date to be able to reduce payment with 2% cash discount would be "today + 10days" (on 15th Sep = 25th Sep)
* In case of Click on Button "Select Invoices to pay" and entering "25th Sep" for due date is the last day we can hold back payment
without loosing possibility to reduce 2 % cash discount. Field due date is in this sense from its functionality the important field to control in case of being in time frame of cash discount (today -> today +10 days) with the field "next payment date" we calculated
with the scheduler if it is the right day to propose the invoice for payment.
* In fact the system shouldn't propose this invoice because control information is like "next payment we will do on 25th Sep. If it is possible to pay not today but instead on 25th Sep keeping ability to reduce cash discount keep the money until then" -> means no proposal
* In case of entering 26th Sep control system for population works like this: 1. today we would have possibility to reduce 2%, on 26th Sept. this wouldn't be the case anymore. Result of control mechanism: Populate this invoice to pay with 2% deduction today! On 26th Sept. we wouldn't have possibility to reduce invoice.

The case is more complicated with two lines or more lines as cash discount (for instance 2% in 10 days, 1% in 20 days). This should be covered with a new calculation of "next payment date", because system changes next payment date after 10 days to "invoice date + 20 days with only 1% discount. 20 + 1 days after invoice date system would calculate next payment date which is identical to "due date".

Please see also once again specification document of Thomas which describes also a use case.

Eventually it would be possible to control proposal only with the field " next payment date" or is this the current situation we
have in this module. This should be good if we calculate seqeuntially the next payment date (starting with the cash discount conditions), ending with the payment term lines.

[Thomas Dreiling (thomas-dreiling)]

fix provided in https://bugs.launchpad.net/bigconsulting/+bug/639698