Implement Support for Scans.tsv

[claytonjschneider]

We have participants.tsv produced, but not a scans.tsv for each session. I personally believe the information in this file is a bit redundant, since it can all be found in the NIFTI headers (except scan date, which is removed for anonymity).

It would be great if on the 1st pass, scans.tsv could automatically be generated from info in the sourcedata DICOM headers, especially now that this optional piece of the BIDS specification is a requirement for NDA uploads to the NIH.

Making the _scan.tsv files after the 2nd pass is already run is a bit of a nightmare, because all of the new filenames.