Add unprocessed Phillips Lab PAM50 levels

[joshuacwnewton]

This PR is an updated duplicate of #4. That PR included a typo in the name "Phillips", so the merged commit was reverted.

Previous PR description, repeated here:

This PR is for safekeeping. It adds the unprocessed Phillips Lab PAM50 levels as a way to backup the files, and not be dependent on the external source repo: https://github.com/PhillipsLab/pam50/tree/main/Spinal%20Cord%20Levels%20NIfTI

Once this PR is merged, then #3 should be updated to remove these files, replacing them with the corrected files from that PR.

This way, we can include a reference to a commit that contains all of the unprocessed level files.