Add cauda equinea label (`60`) to all three point label files for easier lumbar registration

[joshuacwnewton]

These changes will enable lumbar registration by adding a new cauda equinea label (60) to each of the three currently-used label files, to be used as an inferior landmark during registration.

Each of these files corresponds to one of three sct_register_to_template options (-l, -ldisc, -lspinal).

NOTE: Currently, this change will not actually work for the body labels (-l option), because of https://github.com/spinalcordtoolbox/spinalcordtoolbox/issues/4229#issuecomment-1747234925. Right now, the body labels are programmatically generated from the spinal level file, and PAM50_label_body.nii.gz is completely ignored.

The label was added using FSLeyes:

• Turn on Edit mode
• Set "Selection size" to 1 and "Fill value" to 60.
• Press "Change space" button to change to the coordinate space of the selected image.
• Click on coordinate [70, 70, 40] (this corresponds to the lowermost slice of the newly-extended PAM50_cord.nii.gz file)
• Save using Shift+Ctrl+S and choose "Overwrite".

(Side note: The file size increase is quite dramatic for 2 of the 3 files. I used the same FSLeyes-based approach for all three files. However, I will point out that all 3 files are now at about the same file size (85K), so maybe the 2 files were compressed beforehand somehow? I don't quite understand...)

Fixes #26.

[jcohenadad]

We will also need to update the labeling convention

[joshuacwnewton]

I've updated the label value to use 60, as per the comment on the issue. I will also be using this value in the tutorial.

[joshuacwnewton]

I'm going to merge this PR and create a pre-release, because not having the available labels is making https://github.com/spinalcordtoolbox/spinalcordtoolbox/pull/4250 more difficult to review.

This won't be the actual PAM50 release (just a pre-release for review purposes) -- we will create the proper PAM50 release once the outstanding PAM50 spinal level changes have been merged.