Yeah; it's a bit weird, but I think the `network=None` should be explicit.

Another thing that this enables (hypothetically; I haven't tested it) is passing in any Nengo object instead of a Network. Like if you wanted to spin up a simulator just to test out an ensemble; there's no reason you couldn't do that with this because it calls `Builder.build` which works for any object. But, I think until we have use cases, we'll just assume that that isn't possible.