Add octave shift example

[JeffFessler]

This PR provides an example of performing a real-time octave (down) shift using a crude FFT approach.
I did not see any examples that show audio processing of an input stream and writing the results to an output stream and I wanted to have an illustration of that for students in my course.

This is independent of #107 though of course they both work on the examples/ directory.

[codecov]

Codecov Report

Merging #110 (ef0cfcb) into master (78a0a99) will not change coverage.
The diff coverage is n/a.

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  Coverage   97.38%   97.38%           
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  Files           2        2           
  Lines         344      344           
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  Hits          335      335           
  Misses          9        9           

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[bramtayl]

Wow, this looks great! It would good to have it only run for a specific amount of time, so we can eventually run it in CI. The pattern I've used in #107 was

@sync begin
    @async while !done
        # refresh the plot
    end
    sleep(seconds)
    done = true
end

There might be better ways to do this.

[bramtayl]

PS, do you want to join as a maintainer?

[JeffFessler]

PS, do you want to join as a maintainer?

Yes, I would be happy to help in that way. I don't know anything about libportaudio but can help on the Julia side. Thanks!

[JeffFessler]

Ok, I implemented the suggested @async model, which was a nice trick for me to learn.
Perhaps another way would be to calculate the number of loop iterations from the duration and the buffer length and the sample size:
Niters = Int(seconds * sampling_rate / N)

I might slightly prefer that way, because it will look less mysterious to beginnners.
Do you have a preference?

[bramtayl]

Do you have a preference?

Nope, whatever works!

[JeffFessler]

Ok, I went with the loop approach and added a bit more documentation and now it is ready for review.

[bramtayl]

This looks good to me!

[bramtayl]

This looks good to me!