Further enhancements

[DL6ER]

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---

This pull requests add the following convenience features.

• Draw a red point if a survey failed. Remove this red point when starting a new survey.
• Change color of moving point from red to lightblue as we're now using red to indicate failure.
• Do not start maximized. This makes sense now that we support image scaling.
• Disable scanning by default as it takes quite some time and does not give all that much information in the end.
• Make SERVER an optional property to easily skip iperf3 tests in case there is no suitable server.
• Add frequency graph and differentiate between upload and download UDP jitter.
• Update Dockerfile to use patched version of libnl
• Allow starting wifi-survey without any mandatory arguments. Missing items will be asked for with pre-filled interactive dialogs:
  
  
  
• Store used image filename in JSON file so wifi-heatmaps works with TITLE alone. This can still be overwritten when explicitly specifying an image.
• Add optional argument --contours N to draw N contours in the image (with inline labels)
  
• Ensure survey points are drawn on top of everything else when they are used and that contour lines are omitted for uniform data.
  
• Only try to plot for what we have data in the JSON file
• Allow iperf3 server to be specified as ip:port
• Update the README to reflect these changes.

[jantman]

As usual, thank you so so so much for this!