[IMP] account_edi_ubl_cii: handle payment terms

Handle payment terms for UBL as described by the Belgian Peppol
Authority and documented by VBO-FEB.

In case of early payment discount (for instance 2% if payment within 7
days):

* add one document level allowance per VAT rate (this allows to reduce
the tax base amount to take the discount into account)

* add one document level charge exempt from VAT (the amount of this
charge is the amount of the discount the customer can get by paying
early)

* Recompute the document level TaxTotal accordingly

Hence, nothing changes for the invoice lines. Importing the xml is then
convenient since each document level AllowanceCharge will create an
invoice line on top of the existing invoice lines. The total amount of
the invoice will be the amount to pay in case of no early payment
discount. To get the total amount to pay in case of early payment
discount, simply remove the document level charge.

References:
https://openpeppol.atlassian.net/wiki/spaces/Belgium/overview
https://efactuur.belgium.be/sites/default/files/3.%20Cash%20discount.docx



opw-3235769
task-3326963

closes odoo/odoo#132625

Signed-off-by: Laurent Smet (las) <las@odoo.com>