Adding source filename/path in scans.tsv

[adam2392]

Brought up in: bids-standard/bids-specification#906

This would add the source filename/path in scans.tsv and make a corresponding scans.json during write_raw_bids to track the filenames.

We can accomplish this by adding a parameter to write_raw_bids or appending an option to anonymize_dict in write_raw_bids. This would default to 'False' and would be part of the write_raw_bids API.

[adam2392]

This was closed by #887