[FIX] account: tax included

- Create a tax included of 21 %
- Create a product sold 7.00, set the 21 % tax
- Add the product in a SO

Amount w/o tax: 5.79
Amount tax: 1.22
Total: 7.01

We defer the base rounding to the end of the method so that the rounding
doesn't impact the tax computation of included taxes.

Complement of 6e46ba7b

opw-777221