Round quantity before using it on invoice line or stock move

Precision can be lost also with only addition and substraction because of
float, so quantity must be rounded before used on invoice line or stock move.

issue4191
review7621002
(grafted from 719a08d24fab9515b7677b27fde07b42f18a9d9e)

--HG--
branch : 2.8
extra : source : 719a08d24fab9515b7677b27fde07b42f18a9d9e