keep 4D ch_name in separate field

Sorry to repeat  #873, but I still need the original 4D names, and considering your previous comments wrt layouts, I figured the easiest way to conserve the original name is in a separate field.

I'm not sure if this will cause problems elsewhere where `'ch_name_4d'` is not in `FIFF_INFO_CHS_FIELDS`, e.g. if code iterates over key/vals and expects only certain fields. Any suggestions for alternate approach?