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
 - Label plots
ENH: FIX for Freesurfer's mri_surf2surf
API utils:  remove `split_label()` (added to mne)
ENH MneExperiment:  use precomputed morph_matrix for morphing STCs
API MneExperiment:  default value for `inv`