Greyed out underpopulated neighborhoods and fixed borders getting cut off

-> Making underpopulated neighborhood clusters appear blank on the map
Observatory circle, walter reed and rock creek park neighborhood
clusters have fewer than 25 residents each - these should appear as
blank outlines on the map to avoid confusion. Bolling AFB is a margin
call, bit has a few hundred residents, so it can stay.

->Bring hovered neighborhood to front
Right now, the active state of the hover shows a black border, but it
gets cut off if the path is below other paths. Ideally bring that path
up to the top with z-index or d3 equivalent.