ENH MneExperiment:  manage bad channels with files
ENH FileTree:  add cache handlers
ENH TreeModel:  add ._temporary_state context
ENH MneExperiment:  use caching for `evoked-file` and `cov-file`
ENH MneExperiment:  `event-file`

- `raw-evt-file` -> `event-file`;
- cache depending on `raw-file` mtime
ENH MneExperiment:  use caching for `fwd-file`
RF MneExperiment.make_src():  use mne 0.8
ENH MneExperiment.set_root():  use os.path.normpath()
Require mne-python 0.8
DOC:  MneExperiment

.make_report():  require samples > 0
ENH plot.brain:  parallel projection options
API plot.brain:  default to parallel=True
DOC/STY:  multiple comparison correction
API data-obj:  enforce Dataset keys to valid variable names
API data-obj naming:  make auto-generated names valid
RM Factor.project():  same can be achieved with indexing
FIX Factor._child_kwargs():  cases where name is None
API data-obj:  use name=None or name=True
ENH:  ADD Var.split()
DOC testnd