Support BDF/BDF+

[jrevels]

BDF/BDF+ is essentially "24 bit EDF/EDF+": https://www.teuniz.net/edfbrowser/bdfplus%20format%20description.html

We could add a simple bdf flag to the File constructor (e.g. File(io, bdf=true)) and make sure to handle BDF-specific quirks on read/write. We should probably define a simple Int24 primitive type internally for storage, and decode can handle it during conversion to floating point whenever any actual computation needs to be performed.

[palday]

I think BDF also limits some other things (e.g. requires constant sampling rate and includes a status channel), but it's the native format of Biosemi amplifiers, so we wind up supporting a huge class of research equipment. There's also GDF ..... There's a little bit on this over at bids-standard/pyedf#2 .