SR830: Add increment/decrement sensitivity functions.

[spxtr]

I often need to auto range lock-ins mid-sweep. I think it's usually best for instrument drivers to only act as instrument drivers, so I went with as small a change as possible to let me implement autoranging outside of the driver. It would have also been possible to add autoranging behavior into the driver itself, and if y'all prefer that then I'd be happy to update this PR.

The two functions in this change make it possible to do things like the following in my measurement sweeps:

def autorange(srs, max_changes=1):
    def autorange_once():
        r = srs.R.get()
        sens = srs.sensitivity.get()
        if r > 0.9 * sens:
            return srs.increment_sensitivity()
        elif r < 0.1 * sens:
            return srs.decrement_sensitivity()
        return False

    sets = 0
    while autorange_once() and sets < max_changes:
        sets += 1
        time.sleep(srs.time_constant.get())

@StefanD986 @astafan8

[codecov]

Codecov Report

Merging #1380 into master will not change coverage.
The diff coverage is n/a.

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[jenshnielsen]

@spxtr I think your idea is good and it makes a lot of sense to implement this and then make it possible for the user to implement the autorange. Would you be willing to document the feature in the example notebook here https://github.com/QCoDeS/Qcodes/blob/master/docs/examples/driver_examples/Qcodes%20example%20with%20Stanford%20SR830.ipynb

[astafan8]

I agree with the approach described in the description of the PR. Thanks! And it would be indeed great if you could document this feature and how to use it in the driver's example notebook (basically just copy pasting the PR's description with some extra explanatory text would suffice).

[astafan8]

I'd like to suggest reusing the sensitivity parameter, so that duplicated bare SCPI commands are not scattered around the driver (unless really needed).

So, in order to do that, what about the following:

self.sensitivity.get()  # or self.sensitivity()
n = self.sensitivity.raw_value

and for setting the new sensitivity value below, just use the n_to:

self.sensitivity.set(n_to(n + dn))  # or self.sensitivity(n_to(n + dn))

What do you think?

[spxtr]

Done. This does make sense, I didn't know the raw value was accessible in this way.