Basil fix breaks periphery

The recent basil fix for Keithley 24xx series SMUs breaks the periphery class:

The now used current/voltage read functions do not switch on the SMU output before trying to measure. Since measuring is only possible with output on, the query returns an empty string '' if the output is off. In some functions of the periphery class the current/voltage is measured without ensuring that the output is on, for instance when calling power_off_sensor_bias if the output was already off (https://gitlab.cern.ch/silab/bdaq53/blob/development/bdaq53/periphery.py#L251) this case is not considered.

Possible solutions:

• Switch on output before measuring. Not sure if that's a good idea: say the voltage is at some default value & output off, then you start some scan script that wants to ensure the output is off at the beginning...
• Perhaps there is a query for "is output switched on?" so that the 'output off' case could be catched (i.e. drop ramping etc.)
• ...?