FIX MneExperiment:  group argument in make_report and load_test
FIX MneExperiment:  group by exclusion
FIX cell name:  coerce to unicode instead of str
DOC TreeModel.set():  error when trying to set non-str (&STY)
ENH MneExperiment:  more flexible cov setting (API .set_inv())
ENH MneExperiment report:  more information

- intro: more data info
- clusters: include cluster id and value
STY MneExperiment
ENH MneExperiment:  more report info
API MneExperiment.make_report():  expose the include argument