Create `roll_call` module

[egrace479]

This would have a RollCall class that would include check_existing_images and the other pre-download verification functions (pulled out in #56).

• Add hash-based check_existing_images for when --verifier-col is passed. This should raise an issue when there are mismatched filename and hash values to the provided CSV (i.e., a BuddyCheck prior to download).
  • TODO (decision needed): Should we have a flag for non-unique checksums? We could just check for uniqueness of checksums within a directory and have a final duplicate check come out of the post-download BuddyCheck as a note, here are duplicate images you have. Might be better to have it check for duplicates in the validation column ahead of time and just allow a flag --ignore-duplicates to allow for downloading duplicated images under different filenames.