Gradually change TempoClock tempo [ accelerando / decelerando ]

1

This gradually changes the tempo of a TempoClock, each beat of a specified number of beats, and starting at the next bar. Personally, I was not able to heavily distinguish between `.linlin` and `.linexp` for reasonable musical tempos. There may be a mathematically smoother way of handling this at the control rate. This function does not gracefully handle multiple simultaneous invocations.