Pass entity window start dates to ChordReducer

Another PR-a-day. This should hopefully be less controversial.

The main concern will be performance and memory concerns of having `windowFeatures * ChordEntities` now. Not sure there's anything we can do about it.
Now that we're calculating the window dates on the fly there's no requirement that the windows much be an `Array`, but this doesn't necessarily seem like a bad idea. Happy to change `ChordWindowEmitter` if that's preferred.