Cleanup SAMSEG segmentation

[jcohenadad]

SAMSEG segmentation has the following useful classes:

• 258: body
• 165: skull
• 259: eye

We need to create a separate file for each of them, to be able to pre-process some of these labels, eg: smooth the skull one, as it is very noisy, see orange label below:

Motivation for the body class

Even though the body class that @CharlesPageot did already includes the head (see #4 (comment)), the SAMSEG head body is of better quality. See below comparison between the two body labels, with the one from Charles missing part of the head:

[mathieuboudreau]

Done via Slicer in https://github.com/shimming-toolbox/b0-fieldmap-realistic-simulation/blob/4f3c471db542fa9b12f308aaeece401323980965/b0realsim/slicer_scripts/smooth_segment.py