Topic/dayssincereducer

Adds a day's since reducer, as well as a set of interval statistics reducers.
I didn't get time, but would be nice to share this with `GradientReducerSpec`. Don't need to go in another file, I would put it on a companion object in the other spec and just call it from here.

Unrelated, given that `IntervalReducer` is generic there's an argument that says we only need to test it once and not have these three specific (and duplicate) tests here. Up to you though.