Add a low-frequency amplitude modulator

[jbphet]

In phetsims/wave-interference#497 (comment) the sound design team was discussing using a low-frequency oscillator (LFO) to control the amplitude modulation of a sound (in this case, the sound for a moving speaker that is creating a sound wave). This idea has also come up for modulating the sounds used in the "Proportion and Ratio" simulation. We should have a reusable component that makes it reasonably easy to integrat low-frequency amplitude modulation into our sounds.

[jbphet]

This is implemented. @samreid - heads up that this exists and can be used for the wave sound in Waves Intro if so desired. You can see a demo of it in action on the 3rd screen of tambo. Feel free to close this issue once you've had a look.

[samreid]

Thanks, I tested the demo and will keep this in mind.