Created Saver class to save KF states
Created Saver class to save KF states

I didn't like the built in save code that I added to the KF,
as it violates the 1 responsibility pattern, so I created a
separate class to save the KF states.

aitatanit/filterpy,rlabbe/filterpy,aitatanit/filterpy,rlabbe/filterpy,BrianGasberg/filterpy,BrianGasberg/filterpy