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 for_testrunner_2 in 0 seconds
Status Job ID Name Coverage
  External
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failed ERP5.UnitTest-TestRunner2

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passed ERP5.CodingStyleTest-TestRunner2

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failed ERP5.UnitTest-TestRunner2

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