ENH: grand average function #1239 

simple grand average function.

It equalize channels first and calls create the grand average object. 

There is presently no option for interpolation.
indeed. squashing commits might help. @MadsJensen let me know if you prefer to open a new PR. If not, just ```git rebase -i HEAD~7``` and then an editor should open. put an s in each row before each commit that you want to squash. Then close the editor. It will once more open and ask you to confirm by closing.