Solution: use plotting as inner process

- preparation for spawning instead of forking
- fixes issue with message boxes

Not working yet because thread in GUI is missing

- introduce multiple measurement sets in the dut_measuremets
- create one plot per measurement set for time traces
- analysis is showing all sets against temperature/humidity

All notions of VNA, magnitude and phase in prototype.py and the GUI have been
removed and generalised.
This is just refactoring, no functional change. However, the syntax of the
config file has been adapted.