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