added gui to flock model

Besides adding a gui to the model, I also commented out the 400-tick stop condition (to make it easier to experiment with changes in flocking behavior), and removed the patch rect caching, which didn't seem to be used at all (though maybe I'm missing something?).

I was planning to add an isTorus checkbox to the gui, but it looks like we'd need to add a function to do that? Would it be as easy as changing the isTorus variable in the world and all agentsets?

as.dat.gui depends on dat.gui, so I added dat.gui to the libs folder. Does that sound like the right place to put 3rd party dependencies?

Playing around with some of the flocking parameters, I noticed that the primary effect of `minSeparation` seems to be to *prevent* boids from flocking. A high value of `minSeparation` essentially disables flocking, as only the `separate` behavior gets called. On the other hand, with `minSeparation` set to 0, flocking happens very quickly (and the framerate drops, presumably because `align` and `cohere` behaviors get called more often).