Bug 9506: FIX GST values for the basketgroup print action

Bug 9506: FIX GST values for the basketgroup print action

Due to the multi VAT development (Bug 5335), values are not well calculated in
the pdf generated by the basketgroup print action.

Test plan:
- Add one or more basket to a basketgroup.
- Close and print this basketgroup
- Check that different values correspond to values in the basket detail page.

Don't forget to test with different parameters (multiple vat, vendor
include/don't include tax).

Signed-off-by: Mathieu Saby <mathieu.saby@univ-rennes2.fr>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>