ENH plot ndvar:  add Xax and ds arg to all plotting functions
ENH Sensor dim:  add x, y and z properties
ENH mne_experiment:  add .next() method to move through field values
ENH mne_experiment:  more rejection options

- use `kind` entry as primary option
- allow "make" (made by the user but not with rejection GUI)
STY mne_experiment.list_files()
ENH mne_experiment:  add .make_raw() with some defaults
ENH mne_experiment.make_epoch_selection():  automatically specify mad channels
ENH mne_experiment:  add make option to .get()
FIX mne_experiment:  test name should include cov
FIX mne_exp ttest