ENH MneExperiment:  reorganize labels

 - use different files:
	 - annot-file (Freesurfer *.annot)
	 - parc-file (pickled dictionary of label objects)
 - specify labels as string, no `label` state entry
FIX TreeModel.iter():  use field_values from values parameter if key is not in ._field_values
ENH: FIX for Freesurfer's mri_surf2surf