FIX: Add check for EEG average reference

I haven't touched KIT or BTI, but they should still get the warning at the inverse stage if they aren't using an average EEG reference. I think that's safest for now -- we can add EEG ref functionality to those once the use case comes up. I don't know much about those systems, so if it takes more than just calling `self._add_eeg_ref()`, then someone else might need to do it.