ENH combine():  Datasets inherit info (intersection)
FIX load.tsv():  empty string cells
ENH testnd:  ADD clustering to 1-sample t-test
ENH Var, NDVar:  implement unary operations
ENH MneExperiment.show_subjects():  add count and mri columns
ENH MneExperiment.show_subjects():  use current group instead of parameter; add caption
ENH Experiment.show_state():  add `hide` parameter
FIX MneExperiment:  stc_ndvar call
API MneExperiment.add_epoch_stc()

- add option for source space baseline correction
- make etc NDVar name consistent between evoked and epochs
API MneExperiment:  .add_evoked_label() —> .add_stc_label(), handle epochs as well as evoked datasets
ENH MneExperiment.add_evoked_stc():  when morph is requested and all subjects are common_brain, don’t morph
ENH MneExperiment:  add epoch parameter `sub`
FIX MneExperiment.iter():  only include subjects in the current `group`
ENH MneExperiment:  make _derived_labels an attribute
ENH MneExperiment:  _process_subject_arg() to unify combined group/subject argument
ENH MneExperiment.show_subjects():  return Dataset
ENH MneExperiment:  gather ._values from all base classes
RM MneExperiment:  ttests and rm_fake_mris()
ENH MneExperiment:  results file structure
ENH MneExperiment:  ADD `.plot_roi_tc()` and `.plot_roi_tc_subject()`