Bug 29288: Add current_checkouts and old_checkouts methods to Koha::Biblio

Bug 29288: Add current_checkouts and old_checkouts methods to Koha::Biblio

This patch adds helper methods for accessing current and past checkouts
for a given Koha::Biblio object.

To test:
1. Apply the unit tests
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Biblio.t
=> FAIL: Methods are not implemented
3. Apply this patch
4. Repeat 2
=> SUCCESS: Tests pass!
5. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>