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!
I have fixed this for all communicators that are relevant in https://github.com/Galvant/InstrumentKit/commit/9b54c2e066ae761ed783df819fa9859357bff13c