Provide a serial "thread pool" for use in the tasking

[Yurlungur]

PR Summary

PR Checklist

This has come up multiple times before, but currently trying to chase down a throw and getting blocked by the threading. This MR provides a simple compile time option to fake a thread pool and run the new tasking machinery entirely without threading. Simply build with
-DPARTHENON_USE_SERIAL_POOL=ON
and the thread pool will be replaced with a simple queue.

@AstroBarker try running with this branch to debug your issue. @jdolence thoughts on this addition as a debugging tool? I believe @bprather also did something like this. But I'm not sure where that MR went. So I just re-did it.

• Code passes cpplint
• New features are documented.
• Adds a test for any bugs fixed. Adds tests for new features.
• Code is formatted
• Changes are summarized in CHANGELOG.md
• Change is breaking (API, behavior, ...)
  • Change is additionally added to CHANGELOG.md in the breaking section
  • PR is marked as breaking
  • Short summary API changes at the top of the PR (plus optionally with an automated update/fix script)
• CI has been triggered on Darwin for performance regression tests.
• Docs build
• (@lanl.gov employees) Update copyright on changed files

[AstroBarker]

This looks good to me! Worked in phoebus and improved output on crash for debugging greatly.

[bprather]

Looking back I don't think I made my changes into a formal MR, I just hacked out the threading without the niceties here. This is the right way to do it.
Though, someday this may tempt someone into making a ProcessPool using IPC, and I am registering now I want no part of that mess.

[Yurlungur]

Ping @pgrete @jdolence @BenWibking @forrestglines @pdmullen any objections to this MR going in?