adding shm shortcut for simple harmonic motion

I found this useful, not sure if you agree, but I added a shm (simple harmonic motion) global as an easy way to generate harmonic (back and forward) motion.  So when live coding, instead of this:

    scale sin((time/1000) * Math.PI)

You can just write 

    scale shm(1000)

I added it to timekeeper, I hope that's the right place!