Optimised IV curve a bit

This PR includes the following:
* Caleb's PR #174 (required this to work on my Mac)
* Setup of Keithley changed: no more -100 V at the begin on the outlet. But set voltage ranges now. Altered avergaing.
* General speedup by lowering the delays.
* Delay unit in gui/config is now ms (was in 500 ms units - found this a bit unlogic...)
* Another speedup by retrieving voltage/current at the same time, saving roundtrips. Maybe requires a longer delay if users relied on this as a hidden delay ;)
* If the users aborts the measurement on the Keithley, the program treats this as tripped now (you can tell apart in log: message mentions button pressed)
* To do all this, I've added another log level to declutter the output a bit. DEBUGRPC is a bit too talkative if you only want to watch what RS232 is doing.