Fixed unpacking in SR830.trace method

Hi p3trus,

I fixed a small bug in the SR830 class. The ``trace`` method used to fail when unpacking the data points received from the instrument. Example:

```
Traceback (most recent call last):

  File "<ipython-input-56-7781939e7eb8>", line 1, in <module>
    [ch for ch in lockin.trace(1, 0)]

  File "C:\Python27\lib\site-packages\slave\sr830.py", line 455, in <genexpr>
    return (float(f) for f in result.split(','))

ValueError: invalid literal for float(): 2.441425e-004,
```

The changes are minor (see below), so you should be able to pull without any hassle.

Cheers,
Douglas.