[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

When a