ENH: mne_experiment mrisubject handling:

 - use keydefaultdict instead of looking up subjects_dir
 - .subjects_table()
 - .reset() uses .set() instead of updating ._state directly
ADD: Notifier
mne_experiment: ADD .notification
ENH mne_experiment fwd handling:

 - include mrisubject in filename
 - allow different bem files (for fsaverage/proper MRI)
 - don't delete on .make_fwd_cmd(redo=True)
ENH mne_experiment: .rename() and .rm()
RENAME some mne_experiment templates
ENH: mne_experiment single subjects pipeline (load epochs and add epochs stc)
PRUNE: mne_experiment