Bug 18736: Use rounding syspref to determine correct prices in calculations

Bug 18736: Use rounding syspref to determine correct prices in calculations

To test:
Place an order (no tax just for simplicity)
 listprice/rrp = 16.99
 discount = 42%
 quantity = 8
 estimated calculated at 9.85
 but order total is 78.83, but 8 times 9.85 = 78.80
Apply patches, set OrderPriceRounding syspref to 'Nearest cent'
Not order total is now as expected
View ordered.pl and confirm values are correct
Complete order, view invoice and confirm values
View spent.pl and confirm values
Go through acquisitions module and confirm prices throughout are
correct.

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