Bug 36049: Fix price rounding

Bug 36049: Fix price rounding

This patch only fixes price rounding by using Math::BigFloat.

To test:
0) Enable syspref OrderPriceRounding
1) Create an order line in acquisitions with vendor price 18.90 and a
   discount of 5 %.
2) Verify that the total (tax excl.) in the order table on the basket
   page is 17.95.
3) Apply the patch.
4) Reload the basket page.
5) Verify that the total (tax excl.) in the order table on the basket
   page now is 17.96, which is correct.
6) Run: prove t/Number/Price.t

Sponsored-by: Karlsruhe Institute of Technology (KIT)
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>