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?
BTi channel info has both a standard name and a label. The names are the A1, A2, etc hard coded by the acquisition machine but the labels can be set per run, and these labels contain the sEEG electrode names.

`RawBTi.__init__`  can't know what sEEG channels are in the data (per patient and ambiguities between EEG & sEEG names), but the sEEG channel names are only avaiable in `bti_info['chs'][:]['chan_label']`.

Originally, I thought the channels were reordered (brain fart), so I made an extra entry ch_label to have the neuromag name & the bti chan_label.

Now, I see that if I can just get the bti_info dict, those channel names are available, which is why I suggested to drop this change and just have a private _bti_info attribute on the RawBTi object.

Does this make sense?