ENH: Add other options to FIF

[larsoner]

As mentioned by @mkajola in #29 (comment) :

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

• Channel name 15-char limit (ENH: Add channel block information #33)
• Coordinate frame specified per channel rather than being fixed to a single channel type (ENH: Add channel block information #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

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