SMB uncertainty is not fixed at 9 %

SMB uncertainty is 9 % when taking the mean of three RCMs each with 15 % uncertainty.

```python
unumpy.uarray([1,1,1], [15,15,15]).mean()
> 1.0+/-8.660254037844387
```

But when there are only two RCMs, uncertainty grows to 11 %:

```python
unumpy.uarray([1,1], [15,15]).mean()
> 1.0+/-10.606601717798213
```

And the full 15 % when only using 1 RCM.