Support component customization of case.setup clean

I have a usecase for one of our atm components (SCREAM) that I'm not sure how to handle. The buildnml for this component makes some YAML files that are processed when the component is run. buildnml is coded to not overwrite these files if they already exist in the case in order to preserve local modifications. What I want is for these files to be removed when the user does a ./case.setup --clean and regenerated when the user does a ./case.setup --reset . What I am seeing is that CIME has no way to notify a component that a clean/reset has been requested.

Some slack discussion has us in tentative agreement that we could add support for a cleaner script in components that `./case.setup --clean` would look for and invoke if found.