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`
STY MneExperiment.make_besa_evt()
STY:  eellab imports * from lab
ENH fmtxt:  `Section.add_image_figure()` allow adding image from existing file
ENH NDVar:  implement `Dimension.__eq__()`; check dimension when using NDVar index
STY fmtxt
STY EXA:  replace `eelbrain.eellab` with `eelbrain.lab`
WIP MneExperiment:  perform label sign flip for fixed source estimates
WIP: ENH:  add `labels_from_clusters()` (NEED proper smooth values)
ENH NDVar:  make boolean NDVar from comparison
OPT plot:  import plot.brain without importing surfer
FIX MneExperiment:  assign `subject` field values `[]` when `find_subjects==False`
ENH Notifier:  Drop into PDB on exception
API Notifier:  remove message stack (simplify)
FIX NDVar:  implement Dimension.__ne__()
WIP:  OPT nd-anova:  preallocate arrays