Strawman fix for the custom isotopics for testing.

[jakehader]

What is the change?

Why is the change being made?

---

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]

@jakehader Just a reminder of the ARMI PR policy:

armi/doc/developer/tooling.rst

Lines 45 to 49 in bd0b43a

	Don't open until it is ready
	----------------------------
	.. important ::
	Wait until your PR is complete to open it.

We are all extremely busy, and if a PR gets opened before it is ready I will get an email:

• every single time you commit, and the CI passes
• for each and every time a single CI job fails

I love the help, and the interesting in fixing the feature. But in the future, please wait until the feature is ready for final review before opening a PR.

[keckler]

Reminder to remove print statements.

[jakehader]

@jakehader Just a reminder of the ARMI PR policy:

armi/doc/developer/tooling.rst

Lines 45 to 49 in bd0b43a

	Don't open until it is ready
	----------------------------
	.. important ::
	Wait until your PR is complete to open it.

We are all extremely busy, and if a PR gets opened before it is ready I will get an email:

• every single time you commit, and the CI passes
• for each and every time a single CI job fails

I love the help, and the interesting in fixing the feature. But in the future, please wait until the feature is ready for final review before opening a PR.

Thanks for the reminder @john-science - I put the PR here but forgot to make it a draft. This was just intended to provide some ideas for how to resolve the ticket and to put something out there to be stress tested by @keckler and others.

There are failing unit tests because I changed the construct arguments so that needs to be resolved still (probably a poor design to require case settings to build a component). Also, we will need to add more unit tests around custom Isotopics (same comment on the ticket).

[keckler]

I ran some tests on my FFTF model to see if this branch works. A couple points:

1. The keff in my model increased by nearly 5000 pcm to a value that is very incorrect.
2. The predicted isothermal temperature reactivity coefficient looks correct.
3. The fuel density when the model is initialized to Thot is way too high (~10%).
4. The change in density between different Thot values appears to be roughly correct, and the number of atoms is being correctly conserved.

So it seems like this change is half working. Somehow the initialized model is getting the densities very incorrect, though.