Dicussion: Cracking down on not passing a Neuron type

We allowed people to pass ints for neurons for a while now because this was what we did in Nengo 1.4, and we wanted to ease the transition. Well, transition time's over! Sorry! (Also: not sorry.)

@tcstewar wanted to keep this around for representing a "default" neuron type. This PR's also serves as a discussion for how best to do this.

In unit tests, we switch neuron types by doing something like this:
```python
for n_type in [nengo.LIF, nengo.LIFRate, nengo.Direct]:
    with nengo.Model():
       A = nengo.Ensemble(n_type(100), 1)
       ...
```
So, in user scripts, it would be something like
```python
neurons = nengo.LIF  # Change to try out rate / direct mode
with nengo.Model():
    A = nengo.Ensemble(neurons(100), 1)
```
But should we do this some other way? One possibility would be to allow people to use `nengo.Neurons` directly, and allow the builder to choose what neuron type that corresponds to at build time.
```python
with nengo.Model():
    A = nengo.Ensemble(nengo.Neurons(100), 1)
```
Any other possibilities spring to mind?