Key error

Hey there

There seems to be a bug on example code `mondrianforest_demo.py` with `PLOT=True` : 

```
Traceback (most recent call last):
  File "mondrianforest_demo.py", line 71, in <module>
    pred_mean = pred_forest_test['pred_mean']
KeyError: 'pred_mean'
```

`pred_forest_test` only has one key `'pred_prob'`. The code tries to access both `pred_mean` and `pred_var`.

Manuel