Ganga needs defaultSE to be set (Writing data to the Grid)

In relation to the issue https://github.com/ganga-devs/ganga/issues/906 it looks like the UserGuide needs to include instructions for setting
```
[defaults_DiracFile]
defaultSE = 'UKI-LT2-QMUL2-disk'
```
in the users ~/.gangarc for when they use DiracFile for data output in the 'Writing data to the Grid' section.
I'm happy to submit the changes for this if nessasary.
Thanks,
Will.