Encode rhythmic activity (attack density) as ribbon width

[gworthey]

Jesse describes this simply as follows: "In a single given measure, add up the total number of notes in all the voices: that total represents the attack density." Hopefully this confirms your understanding, @curran and @seemantk!

[curran]

Awesome, thank you for the clarification here. The ribbons are coming along over at #71. This attack density computation can fit in there as the windowed computation. We can adjust the parameters to compute for each bar - a step size of 4 and a window interval of 4. We may run into trouble if the mensuration changes mid-song, but this will definitely work to get us started.

[curran]

We implemented the attack density computation using the algorithm described above. Here is the implementation for review.

Here's what the attack density ribbon looks like:

[curran]

Just as an experiment, here's what it looks like if the ribbons are always centered on middle C:

This gives more of a histogram-ish view.

[curran]

This was implemented in #71. Closing.