Keithley 6485 + Keithley 6517b support

[francois-drielsma]

This PR adds support for two Keithley devices:

• Keithley 6485 picoammeter
• Keithley 6517b electrometer

It also includes minor fixes in the Glassman FR support (fixes the set voltage/current queries).

[scasagrande]

thank you for the contribution! can you clean up the git history? and it looks like there are some test issues

[francois-drielsma]

Sure can! This might take me a bit of time as I'm a bit swamped with other things at the moment, but I'll get there.