Fix VNA reading and improve plotting
- Fix VNA readings
-- Load VNA config (and calibration) from file.
The *.znxml file has to be prepared in advance and saved on the VNA.
It needs to set the VNA to CW mode and single sweep.
-- Trigger single sweep and wait for it to complete
-- Fix trace definitions
-- Fix scpi command for VNA sweep mode
-- Frequency and VNA config file are new parameters in .json file
- Improve plotting
-- Plot output changed to pdf
-- CSV file and plot file have the same base name
-- Make showing the plot a command line argument.
The pdf file is always created.
- Add status printout during stable measurements