ENH NDVar:  add .rms()
ENH NDVar:  unify methods for summary over axes &TEST
OPT:  rms function (remove avg reference option)
FIX ClusterDist:  use proper time slice for cropping
WIP ClusterDist:  multiprocessing
DOC testnd:  better error message for invalid kwargs
ENH SourceSpace:  allow .index_for_label() with string
WIP SETUP:  use setuptools upload_docs
ENH ClusterDist:  slices of TFCE results