[FIX] stock_account: compensate rounding error

To reproduce the issue:
1. Create a product:
   - Type: Storable
   - Category:
     - Costing Method: AVCO
2. IN 1000 @ 0.17
3. IN 800 @ 0.23
4. OUT 1000
5. OUT 800
6. Open the inventory valuation of the product

Error: the total value is $-6.00, it should be zero

Once all products received, the standard price is $0.20. Its value
has been rounded because the real value is
`(1000 * .17 + 800 * .23) / 1800 = 0,196666667`
The standard price will create a difference when using the products,
because:
`(1800 * .20 = 360) != (1000 * .17 + 800 * .23 = 354)`
That's the reason why a feature tries to compensate such rounding
errors. So, step 4, when preparing the values of the out-SVL, we
check if there is a rounding error, and we find a difference of $6,
which is correct. However, the difference is above the treshold, so
we will not consider it as a rounding error:
https://github.com/odoo/odoo/blob/3ff51daa93a1d670b8f67f79418d4dd48e94875f/addons/stock_account/models/product.py#L197-L200



Here is the issue: the threshold is based on the outgoing quantity
(1000) while the value difference is based on the whole quantity
(1800). This difference should also be proportional to the outgoing
quantity.

Note: The fix will still not work with a small quantity. The only and
best solution is to change the type of the SVL unit cost into a
float.

OPW-3101374

Part-of: odoo/odoo#108072
Co-authored-by: Adrien Widart (awt) <awt@odoo.com>