[MERGE] from trunk
[IMP] point_of_sale: use the product unit's precision for rounding the weight
[FIX] product: the declaration of decimal precision was done after 
the declaration of uom precision, preventing uom precision from going above 
the default decimal precision. + made the Kg unit precise up to grams by default.
[MERGE] point_of_sale: make the kg unit precise up to grams, and use the unit's rounding factor on the scale screen