[FIX] account_edi_ubl_cii: unit prices should not be rounded

When unit prices have more than 2 digits, it is currently not reflected
in the UBL formats. Consequently, the line amounts are not equal to the
unit price * quantity (assume there is no discount, charges or
allowance) and it raises validation errors: "Invoice line net amount
MUST equal (Invoiced quantity * (Item net price/item price base
quantity) + Sum of invoice line charge amount - sum of invoice line
allowance amount".

To fix this, we no longer round the unit prices.

NB: the decimal accuracy should be set in the settings (otherwise, the
default is 2 digits for unit prices).

See https://docs.peppol.eu/poacc/billing/3.0/bis/#_rounding



opw-3290035
task-3302904

closes odoo/odoo#121508

X-original-commit: d7d41991
Signed-off-by: Laurent Smet <las@odoo.com>
Signed-off-by: Julien Van Roy <juvr@odoo.com>