add gain nodes to oscillators to remove click noise when they are stopped

[venkateshpotluri]

We hear a clicking noise when we stop oscillator nodes corresponding to each point once they are done being played. Adding gain nodes and reducing the volume of each oscillator to 0 in the last 0.015 seconds of playback might remove this click noise. source: Web Audio: the ugly click and the human ear. Though this article does not mention this, we may also have to increase the volume of each oscillator for 0.015 seconds at the beginning of playback to make the transition smooth.