Before this commit:
When returning a transfer in AVCO/FIFO, the return's value is not always
the same as the original delivery. The unit cost for the return was
rounded, which introduced rounding errors.

After this commit:
When returning a transfer in AVCO/FIFO, the return's unit cost is not
rounded, allowing the correct value to be set.

Steps to reproduce:
1. Create a product and change its category costing method to AVCO
2. Purchase the product qty: 1 price: 13.13
3. Purchase the product qty: 1 price: 12.20
4. Sell the 2 product and deliver it
5. Create a return for the delivery
6. The value on the delivery is 25.33, but 25.34 on the return

opw-3358531

closes odoo/odoo#126546

Signed-off-by: William Henrotin (whe) <whe@odoo.com>