accounting: suggest date based on trade conditions in create related payment

This introduce a new utility script Invoice_getPaymentDueDate and removes
the previous attempts, SaleInvoiceTransaction_getDueDate and
TradeCondition_getDueDate that were never used.
4 jobs for feat/create_related_payment in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-TestRunner2

04:30:33

failed ERP5.UnitTest-TestRunner2

06:27:25

passed ERP5.CodingStyleTest-TestRunner2

01:35:56

failed ERP5.UnitTest-TestRunner2

01:49:07