Bug 14094: DDC - Add separators for repeated $a subfields (MARC21)

Bug 14094: DDC - Add separators for repeated $a subfields (MARC21)

Mulitple 082 fields are already separated by |, but multiple
$a in one 082 field were only separated by space, making those
not easy to read.

Patch takes care that the | separator is used in all cases.

To test:
- Catalog a record with multiple 082 fields
- Add one or multiple $a subfields to each
- Verify every single classification is separated from
  the others with a | in staff and in OPAC detail pages

Signed-off-by: Nick Clemens <nick@quecheelibrary.org>
Signed-off-by: Jonathan Druart <jonathan.druart@koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>