Switch to using BIDS as primary data structure

[ErikBjare]

BIDS spec & common principles: https://bids-specification.readthedocs.io/en/stable/02-common-principles.html

• Convert CSV to FIF
• Convert FIF to BIDS
  • mne-study-template seems to err on the produced BIDS, so everything might not be correct.
• Run BIDS validator: https://bids-standard.github.io/bids-validator/
• Use EDF instead of FIF?
  • Nah: Is there a "why we use FIF" document somewhere? mne-tools/mne-python#5302 (comment)
• Restructure existing data folder to use BIDS exclusively (use sourcedata for unprocessed/unconverted files!)
• Test with other devices (Neurosity Crown)

Might differ for different devices. Only Muse S has been tested so far.