engine annotations

[jhanley634]

Add optional type annotations so RecommendationEngine will lint clean. Currently there's 18 lines of noise, some of it related to empty container assignments having ambiguous type. (Is [] a list of str? A list of int? Hard to say, until we hint it.)

This wound up turning into a mini-epic:

• 83-engine-hints, which contains no hints due to distraction of ETL speedup and discovering pymongo issues by running the code
• 88-engine-coverage, which exercises much of the engine code and discovered some difficulties
• future 83-part2-engine-hints, which will use the coverage to safely refactor / hint the engine code