[FIX] stock_landed_costs: account/valuation inconsistencies

When stock landed costs are divided per product unit, inconsistencies
may arise between the real stock valuation and the stock valuation
account. This is likely to happen when several products are bought, but
these products leave the stock one at a time.

A numerical example is the following: a landed cost of 15.00 is applied
to a purchase of 13 units. 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
landed cost 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