[FIX] stock: no early currency rounding when computing average price

[FIX] stock: no early currency rounding when computing average price

It is a common need to set a higher decimal precision
for `Product Price` (i.e. the product cost field) for
high volume / low value items. This may typically
require up to 4-6 decimals for e.g. EUR/USD-based
companies where the currency has 2 decimals.
In that case the product cost should be stored with
full precision without applying the currency rounding.
The appropriate currency rounding will be applied
anyway as soon as a transaction actually uses that
product cost (typically in a SO/PO)

bzr revid: odo@openerp.com-20131104173232-84g115x6ykxoc1rh