Field method refactor for OaiDc and SpringshareOaiDc transforms (#177)

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:
* Refactor base transform class OaiDc to contain get_*
methods for optional fields
* Make 'source_record' the first argument in OaiDc field methods
* Move 'or None' to the return statements of field methods
* Remove 'source_record_id' as param, replace with call inside field method instead
* Raise SkippedRecordEvent
  * OaiDc.get_source_record_id
  * SpringshareOaiDc.get_source_link
* Update unit tests
  * Remove OaiDc and SpringshareOaiDc record_* fixtures

Side effects of this change:
* None

Relevant ticket(s):
* https://mitlibraries.atlassian.net/browse/TIMX-283

.gitignore

@@ -87,6 +87,9 @@ ipython_config.py
 #   intended to run in multiple environments; otherwise, check them in:
 # .python-version
 
+# asdf
+.tool-versions
+
 # pipenv
 #   According to pypa/pipenv#598, it is recommended to include Pipfile.lock in version control.
 #   However, in case of collaboration, if having platform-specific dependencies or dependencies

tests/conftest.py

@@ -89,17 +89,11 @@ def marc_content_type_crosswalk():
     return load_external_config("config/marc_content_type_crosswalk.json", "json")
 
 
-# oaidc ##########################
-
-
 @pytest.fixture
 def oai_pmh_records():
     return XMLTransformer.parse_source_file("tests/fixtures/oai_pmh_records.xml")
 
 
-# timdex ##########################
-
-
 @pytest.fixture
 def timdex_record_required_fields():
     return timdex.TimdexRecord(