[FIX] product: pricelist, uom and price_surcharge

The price_surcharge attribute must be computed based on the reference unit of measure (divided by the factor).
This is to make sure than 12 units and 1 dozen have the same price after pricelist computation (opw 599727).

Added test checking the correctness of pricelist computation based on unit of measures.
[FIX] product_visible_discount: discount with different unit of measures

The computation of the price without pricelist should take care of the unit of measure.
e.g. if computing discount for objects in dozen (on a product with price in unit), returned unit price should be (price*12) where 12 is the factor to go from dozen to unit.
Otherwise the compared prices (with and without pricelist) would not use the same unit of measure and the comparaison would be inconsistent. (opw 599727)