tensor2metric: add option to sort eigenvalues by absolute value
tensor2metric: add option to sort eigenvalues by absolute value

This is useful in non-DTI contexts (e.g. for structure tensor analysis).

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3