Global Inhibition

(Pretty picture in #56)

There's a new Edge build function in the builder which notices `Ensemble -> Neurons` connections.  As we only allow connections of this form if they're global inhibition connections (i.e., `transform=[[k]]*n`) it attempts to create a new global inhibition edge.

When filters are put together for vertices any `GlobalInhibitionConnection`s are ignored.

This'll need cleaning up when the builder refactor is done.