Bug 11644: fix occasional failure to update fund amount due to floating-point math

When attempting to update a fund, if the amount unalloccated for the
fund is equal to the total, you can be prevented from saving. This is
due to imprecise floating point number comparison in
check_parent_total.pl

Test Plan:
1) Create a fund where the amount unallocated is equal to the amount
   unallocated for the budget period
2) Edit the fund, attempt to change the name of the fund
3) Note you recieve an error and cannot save
4) Apply this patch
5) Repeat step 2
6) Note you can now update the fund

Signed-off-by: Sean McGarvey <smcgarvey@pascocountyfl.net>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Bug 11644: (follow-up) add FIXME

Arithmetic with monetary amounts is a *hard* problem -- or if
not hard, at least super-fiddly, particularly since in theory
a Koha database could have to deal with dozens of currencies
if used by a large research library.  This patch adds a FIXME
implicitly commenting on the poor state of affairs of available
monetary calculation modules on CPAN.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>