scil_surface_convert.py only supports FreeSurfer surfaces, update this script to support FSL first subcortical segmentations

[grahamlittlephd]

Currently the scil_surface_convert.py only supports freesurfer cortical surface meshes, however many different formats and surfaces exist.

Need to update the script to be compatible with FSL FIRST subcortical gray matter surfaces.

This will need the application of a few specific transformations in order to go for FSLEYE's viewer space to the space of the image and should be implemented before I forget to transfer this knowledge to scilpy.