Scale the dot radii on zoom

[lindsayplatt]

The dots look super large on zoom right now. We should scale them, so they are smaller when you zoom in. One option to implement this is already outlined in the viz-scratch code here https://github.com/USGS-VIZLAB/viz-scratch/blob/master/svg_canvas_map/target/canvas-click-zoom.html#L157-L158.

[lindsayplatt]

It is half set up in create_circles, because the radii change based on the value of scale but the scale value doesn't actually change right now. Need a way to use k from the clicked function when it zooms to figure out how much to scale the circle radii. Can't just do zoom vs not zoom because some zooms are farther in than others.