Make validate download and unpack a specified study

[rbreslow]

the intended usage for validate is to generate the subject/session label mapping (the CSV output to files/) prior to fully processing for user review. This is useful for debugging when run into missing subjects/sessions &/or when checking DICOM files from external sites for completeness

This would require the study/ies to be downloaded before the CSV can be generated (either manually/separately or if not found locally then should try to download), so right now it fails unless download_unpack_copy is run before validate_info

It would also be useful to be able to specify a specific uuid or list of uuids to validate for this reason