Bug 24252: Add relations to Koha::Account::Line

Bug 24252: Add relations to Koha::Account::Line

This patch adds two new relationships to the Koha::Account::Line object.

* credit_offsets - returns all credit type Koha::Account::Offsets
  related to this Koha::Account::Line.
* debit_offsets - returns all debit type Koha::Account::Offsets related
  to this Koha::Account::Line.
* credits - returns all credits related to this Koha::Account::Line.
* debits - returns all debits related to this Koha::Account::Line.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>