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!
It wouldn't be that hard to make that change for all other communicators, using ``assume_units`` to make sure existing code doesn't break. I'm rather short on time this week, but perhaps I can contribute something to do that soon?