ENH: Add other options to FIF

[larsoner]

As mentioned by @mkajola in https://github.com/mne-tools/fiff-constants/pull/29#issuecomment-733643260 :

This question applies also the other restrictions in the format. It would be good to collect list of those so that it would be possible to evaluate if there is a good way to change FIFF of should there be some entirely different way to go.

@agramfort can you think of others that people are running into? I'll comment here when I think of some.

EDIT: Updated list

• [x] Channel name 15-char limit (#33)
• [x] Coordinate frame specified per channel rather than being fixed to a single channel type (#33)
• [ ] A way of encoding more information / being more explicit about what each item means (from https://github.com/mne-tools/fiff-constants/pull/33/files#r531001859)
• [ ] 2 GB limitation

[agramfort]

well 2 me there are 2 classical complaints:

• limitation of 2GB that requires the logic of split files. We have something that works but maybe in 2020 we could have this.
• length of channel names discussed here