Yeah, it would be good to be able to avoid the call to determine output dimensionality. But if we do this we should do something similar in ensembles, because part of only specifying the input dimensionality is to expose a similar interface for nodes and ensembles.

Always passing `t` and only passing `x` if `dimensions > 0` makes sense to me. Though @drasmuss argued above that changing dimensionality shouldn't make these do different things:
```python
a = nengo.Node(np.sin, dimensions=1)
b = nengo.Node(np.sin)
```
I'm not sure how to reconcile that. In the first case, presumably, you'd want to not use time, but instead compute the sin of the input signal.