The Settings Validators should be in the setting package

[john-science]

Right now, the settings validation tools are in the operator package, which is just strange:

https://github.com/terrapower/armi/blob/6a546dd1ca3a81128e5dcc469a5d49853db2047c/armi/operators/settingsValidation.py#L44

But, we have a whole PACKAGE for settings. Surely this belongs there?

NOTE: This should be doable without breaking the API, by just putting a couple imports in the operator __init__.