Bug 24430: Add ->orders and ->active_orders_count to Koha::Biblio

Bug 24430: Add ->orders and ->active_orders_count to Koha::Biblio

This patch introduces an accessor to the related orders, and a method
that returns the active orders count. The target usage for the count is the
API. In the rest of the codebase, we would just call:

    $biblio->orders->count

To test:
1. Apply this patches
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Biblio.t
=> SUCCESS: Tests pass!
3. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>