ENH MneExperiment:  simplify epoch description

 - only use `epoch` and `rej` in templates
 - move `pad` into epoch specification dict
 - rename epoch selection and evoked dirs
ENH Experiment state management

store_state() an restore_state() methods
ENH MneExperiment:  allow initialization without proper root/subejcts
STY:  MneExperiment
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