1) Suppose a tax of 21% intracomm +100%, -100% applied on 100.0.
The computed price_excluded was 121.0 instead of 100.0 because the
assumption was made the sum of repartition lines factor was always 100.0%.

After this fix, the amounts are always multiplied by the sum of repartition lines factor
in order to reflect correctly the impact of each repartition lines.

2) The rounding is now working for any kind of repartition lines and the consistency
of tax amount regarding the repartition lines is always ensured.
See use cases described by the tests for more details.

closes odoo/odoo#36082

Signed-off-by: Laurent Smet <smetl@users.noreply.github.com>