Add tests to cover several untested sections of code

[riley-harper]

This PR adds tests for several sections of code that were either untested or not tested very thoroughly, focusing especially on the hlink.linking.core module. The changes include a few documentation and type hint updates as well.

• linking.core.comparison_feature.generate_comparison_feature()
  • added a test to check error handling on unknown comparison types
  • added a test for the b_minus_a comparison type
• linking.core.transforms.generate_transforms()
  • added tests for the override_column_a and override_column_b attributes
  • expanded documentation for the attributes available to all transforms, which are override_column_a, override_column_b, set_value_column_a, set_value_column_b, and checkpoint
  • added a test to check error handling on unknown transforms
• linking.core.transforms.apply_transform()
  • added type hints and updated the Python docs, which were incorrect
  • added a test for the when_value transform
  • added a test for the remove_punctuation transform
  • added a test for a particular error condition in the substring transform
  • added a test to check error handling on unknown transforms
• spark.factory.SparkFactory
  • added a smoke test to make sure that the factory can create a usable SparkSession