[FIX] stock_account: account/valuation inconsistencies

When the product price is divided per product unit, inconsistencies
may arise between the real stock valuation and the stock valuation
account. This is likely to happen when a product is bought in a UoM
different from the standard UoM of the product.

A numerical example is the following: a box of 13 is bought for 15.00.
An amount of 15.00 is recorded when the products enter the stock. If the
product leave the stock one at a time, 13 entries of 1.15 are recorded
(15.00/13 = 1.153846... ≈ 1.15), which is then equal to
13 * 1.15 = 14.95. In this case, All the products have left the stock
(stock valuation is zero), but 5 cents remain on the account.

This is of course even worse the higher the ratio is. For example, a
box of 4.00 split into 1000 units sold piece by piece will never be
recorded when a product leaves the stock.

The fix is to record the rounding difference on a specific quant. In the
previous example, instead of adding 1.153846... on the unit cost of the
13 units, we do the following:
- 12 units to which we add 1.15 on unit cost
- 1 unit to which we add 1.20 on unit cost

opw-675222