Consider adding support for arima

It may be good to add support for ARIMA, which is a classical type of time-series model and could be used for epidemic curves.

This has been pointed out by J. Duggan, and used for COVID-19 forecasting there:
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0244173

A lead on existing implementation we may want to wrap:
`forecast::auto.arima()`
