MRG, ENH: Minor speedup of permutation tests (#7784)
MRG, ENH: Minor speedup of permutation tests (#7784)

* ENH: Minor speedup of permutation tests

* flake

* preallocate and assign instead of appending

* add more comments
Teekuningas/mne-python,larsoner/mne-python,bloyl/mne-python,cjayb/mne-python,olafhauk/mne-python,kingjr/mne-python,wmvanvliet/mne-python,rkmaddox/mne-python,kambysese/mne-python,Teekuningas/mne-python,bloyl/mne-python,olafhauk/mne-python,kingjr/mne-python,wmvanvliet/mne-python,drammock/mne-python,pravsripad/mne-python,olafhauk/mne-python,kingjr/mne-python,drammock/mne-python,larsoner/mne-python,wmvanvliet/mne-python,drammock/mne-python,Eric89GXL/mne-python,pravsripad/mne-python,mne-tools/mne-python,larsoner/mne-python,Teekuningas/mne-python,kambysese/mne-python,Eric89GXL/mne-python,rkmaddox/mne-python,mne-tools/mne-python,pravsripad/mne-python,cjayb/mne-python,mne-tools/mne-python