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
• 83-part2-engine-hints, which will use the coverage to safely refactor / hint the engine code
• future 83-part3-download-hints, maybe? (Trying to defer work on those files till after 90 merges.)

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@skyfenton
(Ok, I'm timing out on PR 90 that was approved Feb. 20th. It's Feb 23rd now. I had been deliberately holding back to avoid merge conflicts as 90 touches a very large number of files. I do want to see some new edits merge down by Thursday.)