Adding a settings validator for coupling

[albeanth]

Description

It's easy to set numCoupledIterations > 0 and forget to set looseCoupling: True. If you do this, then the coupling scheme will "run" but o.couplingIsActive() will evaluate to false. This can lead to unexpected results.

armi/armi/operators/operator.py

Lines 1061 to 1063 in 359363d

	def couplingIsActive(self):
	"""True if any kind of physics coupling is active."""
	return self.cs["looseCoupling"] or self.cs["numCoupledIterations"] > 0

This settings validator aims to remedy this.

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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]

This seems like a good use for a settings validator.

Great!