Adding metadata for event overlay

After a brief discussion on PAT-help, we decided it'd be helpful to include metadata for data overlay in the xAOD metadata object. This adds that metadata, as well as adding accessors and setters for booleans that have been stored as chars.

In order to get the unit tests to behave, I had to make explicit the types in the setters and getters. I suspect this is just because we have too many options now, and the compiler is no longer able to automatically tell what we want.