Improve Tax computation
Currently there is 2 types of taxes, percentage and fixed.
Indeed fixed is quite poor because indeed it is a per quantity rate but it is missing the unit of measure.
So we should add a unit of measure on such tax but if not filled the value should not be multiplied by the quantity otherwise the amount must be converted using the unit of measure.
But the problem is that account doesn't depend on product module so it must be managed in account_product module.