TypeError when passing list of values as sigma

`get_tau_sigma` used in many of our distributions fails with a `TypeError` when passing a list of values. We should probably fix this for better user experience:

```python
import pymc as pm:
pm.Normal.dist(sigma=[1, 2])
```
```
TypeError: unsupported operand type(s) for ** or pow(): 'list' and 'float'
```