Creating a one-block test reactor to speed up testing

[john-science]

What is the change?

In this PR I created a test reactor from an existing single test block reactor:

armi/tests/smallestTestReactor/armiRunSmallest.yaml

Using this new test reactor in dozens of tests, sped up our unit tests by 25-30%!

Why is the change being made?

I am just trying to speed up our unit tests. (Close #1736)

On my personal laptop, I saw many unit tests go from between 2 or 6 seconds down to 0.1 to 0.2 seconds.

Perhaps more importantly, we now have the tooling to use across the ecosystem.

NOTE: Not all unit tests can use such a small reactor. Be design, if you are doing something like shuffling multiple assemblies, you can not use the smaller test reactor.

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Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify any new/changed code.

• The code style follows good practices.
• The commit message(s) follow good practices.

• The release notes have been updated if necessary.
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in pyproject.toml.

[john-science]

@opotowsky I reduced the scope of this PR for you.

I moved the runLog.py fix and the Python 3.7 thing to another PR, which Tony got.

Hope that helps!

[opotowsky]

This is amazing! can't wait to use it downstream too!