Move Kalman functionality to ElOceanografo/StateSpace.jl

[GordStephen]

That way TimeModels.jl can just focus on specifying particular time series models (ARIMA-family, structural time series, GARCH, etc) and providing algorithms and a nice interface for parameter estimates. More generally this would avoid duplicating development efforts as well.

Elements that would be removed here and rely on StateSpace.jl:

• [ ] StateSpaceModel type
• [ ] Standalone Kalman filtering (faster than standalone Kalman smoothing for non-EM parameter fitting)
• [ ] Kalman smoothing given a filtered model (currently implemented via Rauch-Tung-Striebel. Not a dependency for other parts of the package)
• [ ] Standalone Kalman smoothing (Currently implemented via Durbin-Koopman. Necessary for EM parameter fitting)

[milktrader]

I'm personally in favor of making TimeModels more modest with (as mentioned above) support for ARIMA, GARCH, and perhaps some tests that haven't found a home in StatsBase.

I have a list of those tests somewhere ...

[milktrader]

https://github.com/JuliaStats/TimeModels.jl/issues/23