mrview: Altered tensor glyph rendering
mrview: Altered tensor glyph rendering

Now display the tensor itself rather than mean particle displacement.
Closes #593.

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