ENH:  support volume source spaces
WIP ENH testnd:  test repr (add mintime etc.)
OPT stats:  _t_rel
WIP t-contrast
AMD t-contrast:  pmin instead of tmin
API cluster tests:  default pmin=0.05
WIP OPT _t_rel
WIP DOC _mne
OPT _Effect.sort_idx()
ENH Interaction: as_effects -> LazyProperty
ENH plot.brain:  ADD copy()
ENH fmtxt HTML:  embed images