Document new GUI Marker Widget

[retiutut]

OpenBCI/OpenBCI_GUI#1174

This allows software markers to be placed in GUI CSV and BrainFlow CSV files using UI Buttons, Keyboard Shortcuts, and a UDP Receiver.

• This should be added to the GUI Widget Guide,
• Outline how the UDP receiver works
• Document that the UDP receiver expects a single float
• Share link to sample Python script that sends UDP marker to GUI
• Add to Networking Output Guide

[retiutut]

#190

• It looks like there a few files where the whole file ended up as a diff for some reason. Can you double check your line endings and try restaging the files?
• I'm not sure how to provide external triggers via something like netcat. Can you provide some information (PR comment is fine) on this? Should the external process listen or host the connection? Does the stream have to be started first?

In the included Python example in NetworkingTestScripts folder, we send a single float over UDP to the specified IP and Port. Though, the float is packed as a byte array.

The GUI will always listen on the specified port, and this does not use multicast. When the IP and Port are changed, the GUI will remake the UDP listener after leaving the textfield or pressing enter. A message will show at the bottom of the GUI and console log. This means the GUI and external process are completely independent and either can be started at any time.

Originally posted by @retiutut in OpenBCI/OpenBCI_GUI#1174 (comment)