Create heuristic for converting phantom data to BIDS

[tsalo]

Given that we don't need to worry about anonymizing phantom data, here's what I'm thinking:

1. Use phantom-specific unique identifiers for sub, if we have that info. We'll need to figure out what the situation there is.
2. Use the scan date for ses. YYYYMMDD of course.
3. Include pictures for the phantoms somewhere in the dataset. Just for reference.
4. Use acq field for different protocols.