HP3456a revisited

All new and improved:
- using the new write_only property factories, 
- gi_gpib timeout property
- rescale units when needed
- sphinx love applied

Let me know if any warts remain!
Yeah, that's pretty much what I was thinking. I'll handle it, no worries. Its a pretty simple change so don't worry about it. Plus don't you have something more important to be writing? haha