Error when data tensorize

Hello,

Thanks a lot for your code. 

I find that during the data tenderizing process, I met the bug:

```
ValueError: 'a' cannot be empty unless no samples are taken
```

I assume that the code in data.py in line 246

```
return [r] + _draw_random(tot, n-1, exclude_indices | set([r]))
``` 

Cause the problem.