ENH dataset:  new save options to replace .export()
FIX dataset:

- eval X in .compress()
- remove special case for when mne.Epochs did not have len();
- add `name` argument to combine()
- __getitem__: allow index for ds.eval()
ENH mne_experiment:  inv state; load evoked

- new 'inv' state that contains complete info on inverse operation;
- improvements to load evoked methods
FIX mne_exeriment:  in .get_templates(), copy() original templates dict before modification
FIX mne_experiment:  set subjects_dir in .add_evoked_stc()
FIX? wxterm:  On startup, show dummy modal dialog, which avoids invisible windows
STY wxterm/wxutils:  cleanup
STY mne_experiment
ENH mne_experiment:  field values treatment; update to `epoch`, `rej`

- var_values -> field_values
- mark field_values and secondary values in templates
- add {rej} to epoch, make rejection modular
FIX mne_experiment.rename(): expand template;  fix common prefix search
ENH mne_experiment.reset():  exclude root by default