ENH mne_experiment:  iteration convenience

- rename .iter_vars() -> .iter()
STY shell
ENH mne_experiment:  add  .iter_range() method
ENH mne_experiment:  add .make_fwd()
ENH mne_experiment:  add .make_link()
API:  move split_label() into `utils.mne_utils`, remove `utils.kit`
ENH split_label():  make source space optional
ENH mne_experiment:  label processing
ENH mne_experiment:  make_morph_map()
ENH mne_experiment:  make mrisubject field values ordered, headed by common_brain
WIP mne_experiment:  request/update field_values individually
ENH mne_experiment.run_subp():  set env
WIP ENH mne_experiment: add group management

- .groups attr
- group field
- load methods for groups other than 'all'
STY mne_experiement:  cleanup
ENH ndvar.__init__():  convert x with np.asarray()
ENH mne_experiment:  plot and res templates
SETUP:  add wxpython py2app fields
STY load.fiff:  remove old functions, relative imports
ENH mne_experiment:  make ttests
ENH mne_experiment.parse_dirs():  always set a valid subject value