ETS: handle seasonal models

[sd2k]

For simplicity (since I only really needed ETS as an MSTL trend model) I ignored all seasonal parts of the ETS model, so it really only handles trend and random variation right now. There are several TODOs littered throughout the augurs-ets crate which need resolving for this to work. It will require some seasonal decomposition stuff which doesn't currently exist yet in Rust to my knowledge.