Allow zoom on dark matter graph

[kadrlica]

Transferring from #9 ...

Is it possible to allow the user to dynamically change the zoom of the dark matter graph? I'm able to change this with the browsers zoom level, but it might be useful to make this part of the webapp. (Also, I expect that the content will only grow, and there may be some reason someone would want to visualize the whole network at once.)

[kadrlica]

Moved to lsstdarkmatter/dark-matter-graph#1