Bug 18736: Calculate tax depending on rounding

Bug 18736: Calculate tax depending on rounding

Marcel's comments pointed out that while I tried to avoid storing
rounded values it is required for tax generation.

This patch makes that change and adds test coverage and POD for
populate_order_with_prices

To test:
Follow plan on other patches, ensure that orders and totals match on the
basket, invoice, and budget pages

prove -v t/db_dependent/Acquisition/populate_order_with_prices.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>