Fix compilation error in src/dwi/tractography/scalar_file.h
Fix compilation error in src/dwi/tractography/scalar_file.h

Fixes compilation error that was introduced in commit f9fc2f2a6 due to ScalarWriter class not automatically inheriting members from templated base class

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