Create a naive seasonal forecaster

[williewheeler]

See https://otexts.com/fpp2/simple-methods.html#simple-methods

For the first pass it's ok to carry forward old forecasts if we run into missing data. For example if the relevant observation from 24 hours ago is missing, then I can just carry forward the relevant observation from 48 hours ago to ensure a prediction. Later we can add support for missing data (producing null predictions). But I think the carry-forward approach should probably be supported as an option anyway.