Cleaning out unnecessary uses of r.o

[john-science]

What is the change?

Here I clean out all the unnecessary usages of r.o.

(I also remove the CustomModifier class, which is entirely unused in ARMI or anywhere downstream. I checked. And it was a major user of r.o.)

Why is the change being made?

This PR is progress towards, but not a complete picture of Jake's ticket to completely remove the Operator from the Reactor; #830.

What this PR leaves is the central usage and problem to solve the r.o problem, the fact that our Interface class just throws around r.o and o.r willy nilly:

armi/armi/interfaces.py

Line 338 in e777af7

self.o = r.o if r else None

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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.

[opotowsky]

Looks good to me!