Model improvement

[Aariq]

Implements non-seasonal ARIMA with seasonality captured by Fourier term as described in #30. It is much faster to fit than a seasonal ARIMA, but still takes ~ 3-10 min per variable. Still a WIP. Need to set up workflow so the auto-ARIMA happens once per year and the daily runs just use refit().

[Aariq]

Almost there. Still to do:

• [x] Skip fitting models and forecasting when there's no data (e.g. soil temp)