Add script to get centerline from SC segmentation

[valosekj]

This PR adds the script we used to obtain the centerline from SC segmentation.

@jcohenadad, please, could you comment on the -method fitseg approach we used:

model_sc_centerline/scripts/get_centerline_from_segmentation.sh

Line 91 in 4d6319e

sct_get_centerline -i ${FILESEG}.nii.gz -method fitseg -qc ${PATH_QC} -qc-subject ${SUBJECT}