Consistenty use root_open, not ropen

At the moment this is used all over the place:
```
from rootpy.io import root_open as ropen
```
If I remember correctly we discussed this briefly in Saas Fee and decided to go with 
```
from rootpy.io import root_open
```
in the rootpy code and docs instead as it is a bit more explicit and doesn't require users to remember such an "import convention".