new command 'mtxedit' to convert transform matrices
new command 'mtxedit' to convert transform matrices

Currently only works if both input and reference images used in the
flirt call have 'standard' strides - i.e. standard orientation in
FSLView. Still working on getting things to work for the more general
case.

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