Tractography::Mapping::MapWriter: Fix maximum voxel-wise statistic
Tractography::Mapping::MapWriter: Fix maximum voxel-wise statistic

Use std::numeric_limits::lowest() to provide the smallest value available for the template type, regardless of floating-point / integer / signedness

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