Bug 12969: Add a subroutine to calculate VAT and prices

Bug 12969: Add a subroutine to calculate VAT and prices

This patch adds a new subroutine populate_order_with_prices in the
C4::Acquisition module.

Its goal is to refactore the VAT and prices calculation into Koha.
All scripts will use this subroutine.

Test plan:
Verify that the prices in t/Prices.t are consistent with the values
listed in the file "Prices and VAT calculation - before" submit on bug
12964.
Verify that
  prove t/Prices.t
returns green

Signed-off-by: Paola Rossi <paola.rossi@cineca.it>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>