Braid: Show class averages instead of individual sample traces

[alexlenail]

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 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.