UserPlugins can't define Parameters

[john-science]

Description

Based on the discussion in ticket #939, I did some research and found the obvious truth that UserPlugins can't (at least yet) define parameters.

Essentially, when we armi.Configure(MyApp()), we get the full set of Parameters from the ARMI App(). THEN, optionally, must later, we can read in a Settings file, that might include a UserPlugin. And at that point, it's just too late to define a Parameter. This is, I'm sure, fixable. But it would demand a high-level re-design of how and when we read plugins and parameters in ARMI.

This has always been a limit of my UserPlugin design. But no one had ever tested it before.

Due to Level-of-Effort, I think it makes more sense to leave this as a limitation of UserPlugins, and anyone who needs this much power should just use a regular ArmiPlugin.

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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.

[john-science]

@drewejohnson @ntouran Just FYI.

[opotowsky]

Most of the work was accomplished in the discussion, so I can't speak to that fully, but I certainly approve of the implementation!