[FIX] stock: UoS quantity in stock.picking

Implements the UoS TODO items on stock.picking.do_partial() to fix #1432.
Add a new method _compute_uos_qty() on product.product to computes
product's invoicing quantity in UoS from quantity in UoM.

The created invoice will use the product_uos of the stock.move, meaning keeping
the quantity specified on the partial picking and the unit of measure of the
original stock.move (e.g. recieving 1 dozen from a 12 unit picking should either
get uos=dozen, uos_qty=1 or uos=unit, uos_qty=12, not a mix of both)

Fixes #1432, opw 611479
[MERGE] forward port of branch 7.0 up to 43db726