Field method refactor for Marc transform (#204)

Why these changes are being introduced:
* These updates are required to implement the architecture described
in the following ADR: https://github.com/MITLibraries/transmogrifier/blob/main/docs/adrs/0005-field-methods.md

How this addresses that need:
* Clean up docstrings for utility methods
* Rename xml -> source_record in abstract methods
* Use kwargs when >3 parameters
* Use tuples to define MARC field mappings

Side effects of this change:
* None

Relevant ticket(s):
* https://mitlibraries.atlassian.net/browse/TIMX-288