Remove double-definition of getVoltage()
Add CMake magic for the HV power supply support package. Just select your
power supply at compile time using e.g.
 $ cmake -DBUILD_HVSUPPLY=Keithley273 ..
to get it built for the device specified
Add empty implementations for the three power supplies currently in
discussion: Keithley 273, Keithley 2410, Iseg