Slight performance enhancement for Const models

See https://github.com/astropy/astropy/pull/3027#issuecomment-59576917
I felt this looked ugly and less obvious, but unfortunately couldn't really find a better way to do it. But I did find things are even faster with a try/except, as in:
```
    try:
        return np.empty_like(x).fill(amplitude.item())
    except ValueError:
        return np.ones_like(x) * amplitude
```