Make `growth_rate` output NA when it doesn't have enough data in local window

[brookslogan]

Currently, extreme values at the edges of time series from partial windows can obscure the scale of "real" growth rates, making the output hard to use without postprocessing. Counterpoint: for the most recent data, maybe we'd want to try to get by with a partial window. Countercounterpoint: partial windows would have different meanings and behavior than the main growth rates with full windows, and the latest (& smallest) windows are unacceptably sensitive and would need to be filtered anyway.