NanoVNA fixes

[miek]

I've been trying to use the NanoVNA support included in the recent VNA overhaul, but I ran into lots of basic runtime errors. Looking back over #744, I'm not sure if anyone with a NanoVNA was able to test it before merging, so that probably explains it.

I've gone through and fixed everything I could find to get it functioning, doing my best to keep to the API that was there. I've been testing with a LiteVNA (which implements the NanoVNAv2 protocol) using the following script:

import matplotlib.pyplot as plt
import skrf
from skrf.vi.vna.nanovna import NanoVNA

nv = NanoVNA('ASRL/dev/ttyACM0::INSTR')
print(nv.device_info)

nv.freq_start = 1e6
nv.freq_stop = 6e9
nv.npoints = 201

s11, s21 = nv.get_s11_s21()
n = skrf.network.four_oneports_2_twoport(s11, s21, s21, s11)
n.plot_it_all(n=0)
plt.show()

and it seems to be working well with this set of changes:

(uncorrected output of course)

[cafeclimber]

Thanks for this! Someone had submitted a PR to my fork but I didn't get the chance to merge it before the PR into upstream. I'll take a look at this in a bit!

cafeclimber#1

[jhillairet]

@cafeclimber how about this PR?

[jhillairet]

Thank you @miek

[cafeclimber]

Hey sorry about not having the chance to review this PR. I know it's merged now so I'll try my best to take a look this week and submit PRs if I find anything :) thanks for the contribution, @miek