Allow to define the date used to compute payment term date

As discussed in: https://discuss.tryton.org/t/payment-terms-based-on-shipment-or-another-date/3566/19?u=pokoli

assigned to @pokoli

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New review292961002 at https://codereview.tryton.org/292961002/#ps322911002

review292961002 updated at https://codereview.tryton.org/292961002/#ps322921002

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review292961002 updated at https://codereview.tryton.org/292961002/#ps333611002

review292961002 updated at https://codereview.tryton.org/292961002/#ps353191002

review292961002 updated at https://codereview.tryton.org/292961002/#ps341231003

New changeset 2816523d7ceb by Sergi Almacellas Abellana in branch 'default':
Add payment term date in invoice
https://hg.tryton.org/modules/account_invoice/rev/2816523d7ceb

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closed

New changeset 7e31917c4456 by Sergi Almacellas Abellana in branch 'default':
Add payment term date in invoice
https://hg.tryton.org/tryton-env/rev/7e31917c4456

mentioned in issue #10181 (closed)