Add support to read and update wifi guest network settings

[frimtec]

Dear @ties

Thanks a lot for the useful compal library.

The only thing I'm missing, is the possibility to update wifi guest network settings. I implemented the feature and tested it with my compal modem.

It would be very nice if you could integrate this feature into your library.

If there are issues with my python style, please forgive me, this is my first python code ;-)

Best regards, Markus

[frimtec]

I will fix the tests first and retry again ;-)