ENH to_latex mi index will use & sep for levels
follow up of #7982

_One question I had was the use of `{ }` (is that a convention? I removed it and it seems to compile ok.)_

Note: should test MI cols and MI index. Ah cripes... the names of the column Index don't print, not did they before...

merge when you are happy witht his
 eeep, will have to fix a bug I found in this/`to_latex` (with index.names and column.names)!
