Bug 24463: Add a basket_group relationship for baskets

Bug 24463: Add a basket_group relationship for baskets

This patch aims to bring consistency between the accessor name and the
underlying relationship to ease things when using it from the API.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Acquisition/Basket.t
=> SUCCESS: Tests pass! No behavior change!
3. Sign off :-D

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