Bug 37120: Add heading subfields for 647 (MARC21)

Bug 37120: Add heading subfields for 647 (MARC21)

This patch updates the default MARC21 bibliographic framework to add missing
subfields to field 647.

To test:
1. Apply patch and reset_all
2. Go to Administration > MARC bibliographic framework
3. Click Actions next to the default framework and choose MARC structure
4. Search for field 647
5. Click Actions > View subfields (or Edit subfields)
6. Check that subfields match the documentation
   https://www.loc.gov/marc/bibliographic/bd647.html
   (note that subfields $4 and $e are added in bug 37114)

Added subfields are:
- a - Named event (NR)
- c - Location of named event (R)
- d - Date of named event (NR)
- g - Miscellaneous information (R)

7. Optional: run the framework test in Administration > MARC bibliographic
   framework test
   --> All should be OK

Signed-off-by: Phil Ringnalda <phil@chetcolibrary.org>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>