Constrained zoom for map

[Tanner]

I'd like to add constrained zoom to the map so that you cannot pan off the map.

There is a example on bl.ocks, but there is some odd math that I'm not able to convert over to our visualization.

It's not too much of an issue, but it is a "it would be nice" feature.

[sabinesin]

Good idea. We can put that off for later if we can't find other things to add