dwi2mask: Fix compilation on Mac clang
dwi2mask: Fix compilation on Mac clang

Image value helper needs to be explicitly cast to floating-point, otherwise an appropriate template for std::isfinite() cannot be found.

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