Move ap and covs for BasicKalmanFilter to types.jl
Move ap up to make it consistent with types.jl
Remove old-style sigma point methods
Remove an AdditiveUnscentedKalmanFilter constructor that we don't need
Move all the additive unscented Kalman filter stuff into one file
Add some (hopefully) helpful comments
Rename types to bkf.jl since it just implements the BasicKalmanFilter