Keithley6500 does not set mode correctly

[haroldmeerwaldt]

The Keithley6500 driver does not set the mode correctly.

Steps to reproduce

from qcodes.instrument_drivers.tektronix.Keithley_6500 import Keithley_6500
keithley_1 = Keithley_6500("keithley_1", address="TCPIP0::192.168.2.105::inst0::INSTR")
keithley_1.mode('dc voltage')

Expected behaviour

The mode on the instrument to be set to DC voltage

Actual behaviour

The instrument shows a message on the front panel that the parameter value should be specified as a string. In Python, the commands are executed without exception.

System

Windows 10

If you are using a released version of qcodes (recommended):
0.31.0

Following the manual:
https://download.tek.com/manual/DMM6500-901-01B_Sept_2019_Ref.pdf
the solution is simply to add quotes around the mode value in the command.

Related to #1541

I will add a PR shortly.