Make a subdirectory of the temporary directory per-runset

[WardBrian]

Submission Checklist

• Run unit tests
• Declare copyright holder and open-source license: see below

Summary

Closes #568. When no output directory is specified, each runset creates its own subfolder in the _TMPDIR directory to avoid name clashes. This has also affected fbprophet.

When the output directory is specified, name clobbering will still occur - we assume the user knows what is going on in that case.

Copyright and Licensing

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Simons Foundation

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• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)

[WardBrian]

I should note that this technically makes the RunSet object truly thread-safe when used with the default directory, since tempfile.mkdtemp notes

There are no race conditions in the directory’s creation. The directory is readable, writable, and searchable only by the creating user ID.

[ahartikainen]

Why the filenames can't have microsecond accuracy?

[ahartikainen]

Or uuid4 value per run?

[WardBrian]

They could (and the user could already specify that) but that's not really thread safe, since it's still possible (though vanishingly less common) to get two file sets with the same time. This approach is nice because it keeps the files from each run in a parallel folder, if for some reason you ever needed to navigate to it

[ahartikainen]

Those are solid answers.