ENH: Export to BDF

[larsoner]

Mostly putting here as a placeholder, I expect most discussion to happen in the-siesta-group/edfio#62.

In short, we probably want to be able to export to BDF because this will be preferred by EMG-BIDS and thus soon MNE-BIDS. We should either add support to edfio for writing BDF (my preference), or use PyEDFLib (which we recently abandoned for edfio!) for writing BDF.

[larsoner]

cc @cbrnr since you might have ideas/thoughts about these formats and libraries

[cbrnr]

I've added a comment over at the linked issue.