Allow to set numbered_at after posting
From https://discuss.tryton.org/t/how-to-unpay-invoice-in-order-to-cancel-and-reinvoice-third-party-and-credit-payments-made-by-a-manual-move/2047
It the module is activated after the posting of an invoice, this invoice does not have the numbered_at filled so if Invoice.process it triggered it will try to write it. So we must allow to write on this field after posting.