hange Uom.round into an instance method.

issue4423
review19661002

stock.py

@@ -59,7 +59,7 @@ class Move:
                         'uom': uom.id,
                         }))
             remainder -= quantity
-            remainder = Uom.round(remainder, uom.rounding)
+            remainder = uom.round(remainder)
             if count:
                 count -= 1
         assert remainder >= 0