Added reporting of fitting error.

[capn-freako]

Fixes #17

[capn-freako]

It's a little difficult to review these changes to Jupyter notebooks, using the built-in GitHub diff engine.
So, here's the code I've written, to calculate the fitting error:

_f = ntwk_dd.f
sig = ntwk_dd.s21.s[:, 0, 0]
Hs = vf.get_model_response(0, 0, _f)
mag_err = np.abs(Hs) - np.abs(sig)
rmse = np.sqrt(np.mean(mag_err**2))
rmss = np.sqrt(np.mean(np.abs(sig)**2))
print(f"Average fit SNR: {20 * np.log10(rmss / rmse):.0f} dB")

Is that acceptable?