KSP: smoothing for _set_engine_par

In my experience the _set_engine_par isn't usable for real-time effects since repeatedly calling it causes zipper noise (the primary use seems to be switching between presets). Instead one has to resort to controlling parameters by using MIDI CC and the set_controller function, but that has two problems: much less precision and potential conflicts with MIDI CC configuration made by the user at a later time. Hence it would be good to be able to pass a smoothing time parameter to the _set_engine_par function.