Added HP3456a System Multimeter
Rewrote HP3456a to use writeonly bool and enum_properties

- removed private members that shadowed parameters set on the hp3456a
- added "setter" documentation to "getter" properties for sphinx
- added HP3456a to sphinx apiref/hp.rst
- removed a gi_gpib.timeout bug
Merge branch 'dev' of https://github.com/Galvant/InstrumentKit

Conflicts:
	instruments/instruments/abstract_instruments/comm/gi_gpib.py

My solution was only taking the gpib timeout into account, not the
select timeout on the computer side. Steven has solved that in the
dev branch.
Merge pull request #45 from dijkstrw/dev

Merge HP3456a from dijkstrw