Add `isOn()` to the SCPIPs class
This MR adds isOn()
to SCPIPs.cpp
and SCPIPs.h
in the same way it is implemented in Keithley24XX.cpp.
I have tested it only with our R&S HMP4040, but ":OUTPUT?"
seems to be common for all the SCPI power supplies. It performs according to the HMP4040 Programmers Manual and the function works fine.