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
review13551002
(grafted from b1e1adc84de0956c8d27b26349e7c2ac7599925e)

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branch : 2.6
extra : source : b1e1adc84de0956c8d27b26349e7c2ac7599925e