Bug 14541: Do not truncate tax rate values

Bug 14541: Do not truncate tax rate values

Tax rates are stored in decimal(6,4) fields which means that 4 decimals
are allowed.
If a tax rate is 8.42%, it is stored as 0.0842
If a tax rate has more precision than that, Koha won't deal correctly
with it. We will need to update the DB structure.

With this patch, the tax rate will be displayed with the same precision
as in the DB. So if you enter 8.42, you will see 8.42% instead of 8.4%
without this patch.

Test plan:
Do a full acquisition workflow with a tax rate like 8.42% and confirm
that it is correctly displayed.

Signed-off-by: Alex Buckley <alexbuckley@catalyst.net.nz>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>