add voxel segmentation info
        
        Parameters
        ----------
        
        segfile : str
            Path to nifti file with segmentation info (e.g. XXXX_aseg.nii.gz)
        
        MRSfile : str
            Path to MRS nifti file

def voxel_seg(self, segfile, MRSfile):
        """
        add voxel segmentation info
        
        Parameters
        ----------
        
        segfile : str
            Path to nifti file with segmentation info (e.g. XXXX_aseg.nii.gz)
        
        MRSfile : str
            Path to MRS nifti file 
        """
        total, grey, white, csf, nongmwm, pGrey, pWhite, pCSF, pNongmwm =\
            fs.MRSvoxelStats(segfile, MRSfile)
        
        self.pGrey = pGrey
        self.pWhite = pWhite
        self.pCSF = pCSF
        self.pNongmwm = pNongmwm