Gaussian1dModel.stddev is incorrectly used

While trying to fit some data, I noticed that the ``Gaussian1DModel`` is wrong:

```
    def eval(self, x, params):
        return params[0] * np.exp((-(1/(params[2]**2)) * 
                                                (x-params[1])**2))
```

it should be:

```
    def eval(self, x, params):
        return params[0] * np.exp((-(0.5/(params[2]**2)) * 
                                                (x-params[1])**2))
```