Braid: Show class averages instead of individual sample traces#6

Although the point of Braid plots is to show the structure of the dataset, rather than summary statistics, people _want_ those summary statistics sometimes, and I believe Axial shouldn't take a position of limiting users' ability to plot, even if we believe those plots might be misinformative. This feature aggregates samples by some metadata attribute, and plots a single trace along their averages. The halos in this case represent standard deviations, and will need to become [areas](https://github.com/d3/d3-shape#areas) instead of lines. Bonus points if you have double halos with gradation for 1-2 standard deviations from the mean. The selection (html) UI component for this feature is mostly already there, so most of the work is in the JS.