Add MRI postprocessing of SPM12 outputs as following Huang2013

[harmening]

Add postprocessing of SPM12 segmentation masks following Huang2013.

Starting from SPM12 segmentation, simply do

import cedalion.geometry.segmentation as cgeoseg 
mask_files = cgeoseg.segmentation_postprocessing(seg_datadir)

and mask_files contain the postprocessed segmentation_masks following Huang2013 and Harmening2022.