incorrect treatment of sigma??

Hi,

I suspect in your CIR model you are forgetting to scale the brownian motion term. If I am reading your code correctly, instead of:

```python
dr = K*(theta-rates[-1])*dt + sigma*np.sqrt(rates[-1])*np.random.normal()
```

I would have said:
```python
dr = K*(theta-rates[-1])*dt + sigma*np.sqrt(rates[-1])*np.sqrt(dt)*np.random.normal()
```

What am I missing?

-VV
