[FIX] account_anglo_savon: round prices according to account precision to avoid getting unbalanced move

The standard move_line_get from account.invoice returns prices rounded (price_subtotal field as account precision) while the anglo-saxon module could return dict with non-rounded prices.
With amounts with bigger precision, we could get a difference of a few cents when comparing the debit/credits and then not be able to balance the invoice (opw 597208).