Add example Notebook for usage with NanoVNAV2

[werwaswarum]

Hi,

I bought a NanoVNAV2 and want to use it in jupyter Notebooks.
However, I realized that the nanovna.py has subclasses for the nanoVNAV2, but there is no corresponding Notebook working with the nanoVNA. Indeed to make it work I had just to change the import statement:
from nanovna import NanoVNA
to
from nanovna import NanoVNAV2

Anyway I added example Notebook that works out of the box. I hope my contribution is helpful and will be accepted.

Maybe you prefer to have a single notebook as example?

Best regards,
Lars