`dond` finishes but the measurement in reality is incomplete (for Agilent N9928 VNA)

Hi,

I'm not entirely sure if this is a bug from qcodes side or the driver for said VNA but I noticed that often times when running a `dond` sweep, it completes it on the code side but when referring to the measuring instrument, it is still incomplete.

One particular example is like so: I've set about 1000 averages for a VNA measurement and when setting the same with qcodes and running a `dond` sweep, I often find the measurement completes in about 5 mins (code side) whereas when looking at the VNA, the measurement is nowhere near complete.

The above figure gives a decent idea of the issue. It's clear the measurement isn't complete but `dond` still shows that it indeed is.

I'd appreciate any help on this matter. Thanks.

Start with the dond sweep flow and the Agilent N9928 VNA driver, focusing on how measurement completion is detected. Reproduce the mismatch using a sweep with many averages and compare Qcodes completion with the instrument's actual state. Done means dond does not report completion until the VNA measurement is complete.

Written by the indexing model from the issue text.