Add set_dac_unit method to D5a instrument driver

[qSaevar]

Changes proposed in this pull request:

• Add set_dac_unit method to D5a instrument for chaning the units and gain between volts and millivolts.

@astafan8

[astafan8]

Once that comment of mine is responded to, i'll approve.

But actually - i would really love the driver to be refactored so that there is a single "variable" that knows whether volts or millivolts are used. Now there is ._gain, ._mV, and the dac_unit parameter. But i leave this refactoring up to the author.

[codecov]

Codecov Report

Merging #1582 into master will not change coverage.
The diff coverage is n/a.

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[astafan8]

Great, thanks for removing self._mV.

I am sorry, now I noticed something that I haven't seen before. Why is set_dac_unit a parameter? Why is it not a normal method of the driver? What is the benefit of one .get()-ing the set_dac_unit parameter?

If there's none, then i'd just make it a bound method (set_dac_unit name is fine). and in the body of that method (which will be mostly copy-pasting _set_dac_unit), for validating the input argument the same validator Enum('mV', 'V') can be used - just call it's .validate(...) method with the set_dac_unit's input argument.

[qSaevar]

@astafan8 as there is no real need for .get()-ing the set_dac_unit parameter I converted it to a method as you suggested.

[astafan8]

Great, thanks!

i took the initiative to make the gain value selection logic more explicit than "if-else" stuff. Do you mind?

Once the checks say "green", i'll merge.

[qSaevar]

No, I don't mind. Thanks