lagIn numChannels bus lag

Smooth a control rate input signal.

> withSC3 (\fd -> do { send fd (c_set [(10, 200)])
>                    ; play fd (out 0 (sinOsc AR (lagIn 1 10 1) 0 * 0.1))
>                    ; threadDelay 500000
>                    ; send fd (c_set [(10, 2000)]) })