OPT ClusterDist:  multiprocessing
ENH ClusterDist:  slices of TFCE results
DOC testnd:  better error message for invalid kwargs
ENH SourceSpace:  allow .index_for_label() with string
FIX plot.topo:  vmax & vmin kwargs
FIX plot.topo:  **layout kwargs
ENH SelectEpochs GUI:  GA button to plot Grand Average
FIX SelectEpochs:  key-press over no axes