ENH plot.brain (API):  use new PySurfer features

 - use PySurfer Brain as top level return value
 - guess `smoothing_steps` based on src grade
 - ADD plot.brain.cluster()
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`
WIP MneExperiment:  perform label sign flip for fixed source estimates
STY:  eellab imports * from lab
ENH:  add `labels_from_clusters()`
WIP labels_from_cluster:  NEED proper smooth values
STY MneExperiment.make_besa_evt()
ENH fmtxt:  `Section.add_image_figure()` allow adding image from existing file
ENH NDVar:  implement `Dimension.__eq__()`; check dimension when using NDVar index
AMD
STY fmtxt
STY EXA:  replace `eelbrain.eellab` with `eelbrain.lab`