[DynareDev] Fast Kalman algorithm

I just pushed an implementation of Ed Herbst fast Kalman filter. It is available with option fast_kalman_filter For large models, it gives huge speed improvement. Even for fs2000, it gives some speed improvement. After more of us experiment with it, we could probably make it the default. I'm not entirely convinced by the syntax of the option. There is no way currently to turn it off. I thought about another value for kalman_algo but the the organisation of that option, too complex, should be revised. Tell me what you think Best