Add final set of DspaceMets field methods (#193)

* Add final set of DspaceMets field methods

Why these changes are being introduced:
* Finish refactoring DspaceMets to use field methods

How this addresses that need:
* Add field methods and associated private methods for contributors, dates, edition, file_formats, format, identifiers, languages, links, numbering, publishers, related_items, rights, subjects, and summary
* Add unit tests for new field methods
* Update create_dspace_mets_source_record_stub function to use different inserts

Side effects of this change:
* None

Relevant ticket(s):
* https://mitlibraries.atlassian.net/browse/TIMX-286

* Shift comments to docstrings