Refactoring loadTestReactor to pickle multiple test reactors

[john-science]

What is the change? Why is it being made?

Essentially, when loadTestReactor() was first written, there was only ONE test reactor in ARMI. But now there are all kinds of them in ARMI and in downstream ARMI projects.

This PR helps cache/pickle all such test reactors. Though it provides a method to not do that, say for instance if you know that a certain test reactor will only ever be used in one test. You can save the RAM by not pickling it.

close #2333
close #1636

SCR Information

Change Type: trivial

One-Sentence Description: Refactoring loadTestReactor to pickle multiple test reactors

One-line Impact on Requirements: NA

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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 documentation is still up-to-date in the doc folder.
• The code style follows good practices.
• The dependencies are still up-to-date in pyproject.toml.

[opotowsky]

Oh this is great! I especially love that pickle is now being spelled properly

We know this gives a speedup, but we need to monitor memory too since we are pickling so many reactors (depending on the repo). I will run testing tomorrow for memory use.

[opotowsky]

Obvi we need to wait to merge due to downstream impacts, but I enthusiastically approve of this approach. In the meantime, a couple of comments.

[opotowsky]

This looks great! We have to wait a week to merge so I'll come give a green check when we are ready. But I don't see any more changes needed unless something unexpected shows up in downstream testing.