Check that every file listed in scans.tsv actually exists

[rwblair]

trafficstars

bids-standard/legacy-validator#971 bids-standard/legacy-validator#976 Check that every file in a scans.tsv actually exists. Right now, we only look at all existing files and make sure they are listed in scans.tsv (kind of the other way around)

to do:

• collect all "scans" (i.e., NOT the scans.tsv! ... either nii.gz, or .fif., or a vhdr+eeg+vmrk triplet, or a .ds directory, ...) at a subj/session level
• check that IF a scans.tsv file exist, all of the scans are entered in there