Update embedded GUI example

[bmorcos]

The net.ipynb example still uses the deprecated IPythonViz, This should probably be updated to the new InlineGUI

Just noticed this when @ikajic posted the example in the summer school.

[arvoelke]

Relatedly, the InlineGUI isn't actually mentioned anywhere in the documentation. I think it should be listed in the main README under "Running Nengo GUI" as this is potentially a very common workflow for people in the scientific Python community:

from nengo_gui.ipython import InlineGUI
InlineGUI(model)

or

from nengo_gui.jupyter import InlineGUI
InlineGUI(model)