Improved tooltip display for continuous color-bys

[jameshadfield]

For continuous traits we bin the range of observed values into bins (or the JSON can define the bins), and display a single legend swatch for each bin. In a normal tree, hovering over the legend swatch higlights the tips whose values fall into the corresponding bin, but we don't actually convey the range of the bin but we probably should (e.g. via a small pop-up over the legend):

In streamtrees this is worse because the ripples correspond to these bins, and when hovering over the ripple we see a point value rather than the range of values collected into the ripple:

(As a bonus, we should use the color-by title ("ACE2 Binding vs BA.2") rather than the key!)