Bug 17976: Add overdue_fines accessor to Koha::Checkout

Bug 17976: Add overdue_fines accessor to Koha::Checkout

This patch adds a new filtered relation accessor to the Koha::Checkout
object to return just overdue fine type account lines related to the
checkout.

Whilst this is easy to chain in code, it's not available via TT and so
this accessor is useful for the notices case.

Signed-off-by: Emily Lamancusa <emily.lamancusa@montgomerycountymd.gov>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>