Move imports to method to avoid circular import issue

[opotowsky]

Description

This import caused a circular import that was hidden for a lot of us, partially because so many unit tests call armi.configure() (or app of choice) or because case runs already have an application configured. Thanks to @alexhjames for finding this and being patient while we figured it out!

This resolves the issue found in #1184, but I don't think it resolves the issue itself, since it is broader than this little fix.

I do not believe this is unit testable; willing to be proven wrong.

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Checklist

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

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

• The release notes (location doc/release/0.X.rst) are up-to-date with any bug fixes or new features.
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in setup.py.