[FIX] purchase: prevent rounding during currency conversion

With the settings:
- Anglo saxon accounting
- real price on product, perpetual accounting,
- multicurrency
- a product with a very small uom (like gram or kilograms and you want to sell many of them, like 500kg)

Flow: purchasing something in another currency

Before this commit, the rounding of the unit price took place right at the conversion level
which led the rounding errors to be amplified by the quantities when the total price was computed

After this commit, the conversion is done on the unit price but is not rounded yet
Only the prices are rounded, as late as possible

OPW 742293

Closes #17015