Why these changes are being introduced:
* This is the initial structure for the Aardvark transform class. The class will be expanded with new methods in subsequent commits.

How this addresses that need:
* Add jsonlines to Pipfile
* Add fixtures for aardvark and generic JSONLines files
* Update argument type hinting for Transformer and JsonTransformer classes to clarify expected content types
* Update JsonTransformer.parse_source_file method to use jsonlines library
* Add Aardvark class with get_main_titles, get_source_record_id, record_is_deleted (in progress), get_optional_fields (in progress), and get_subjects methods and corresponding unit tests

Side effects of this change:
* None

Relevant ticket(s):
* https://mitlibraries.atlassian.net/browse/GDT-54

* Update json_records fixture to aardvark_records for more accurate unit tests
* Rename Aardvark > MITAardvark to unify terminology across repos
* Update get_main_titles method to reflect it is a required field
* Update Aardvark method docstrings to provide greater context
* Add Transformer._transform method to minimize code duplication between JsonTransformer and XmlTransformer methods