Consolidate d3 tooltip implementation

[chrisknoll]

We currently have 2 implemntations of tooltips on our d3 graphs: d3-tip (seen on most of our graphs) and popover from bootstarp (seen on the tooltips on our treemaps for condition, condition era, etc). The bootstrap one was applied to the treemaps (d3 tip was the origional implementation) because the tooltips from bootstrap remain in the viewport, while d3tips just pop up (and can be obscured).