Bug 13321: Fix the prices calculation method

Bug 13321: Fix the prices calculation method

Well, we have finally arrived here \o/

The method where the prices are calculated uses the equations listed on
the wiki page (http://wiki.koha-community.org/wiki/GST_Rewrite_RFC).

The ecost is calculated from the rrp (using the discount and the tax
rate). That's why we removed the ability to edit this value.

That's why we remove the ability to edit the ecost on ordering in a
previous commit (bug 12840).

The total is now calculated in the scripts. That's why this patch
removes lines in the test file.

In C4::Acquisition::populate_order_with_prices, the calculation on
receiving must depend on the 'invoiceincgst' supplier parameter, and not
listincgst (which is used on ordering).
It also removes the rounding errors, now we store "exact" values in DB
(10^-6).
The values will be displayed using the Price TT plugin it will round the
values for us.

Signed-off-by: Laurence Rault <laurence.rault@biblibre.com>
Signed-off-by: Francois Charbonnier <francois.charbonnier@inlibro.com>
Signed-off-by: Sonia Bouis <sonia.bouis@univ-lyon3.fr>
Signed-off-by: Sonia Bouis <koha@univ-lyon3.fr>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>