NIfTI: handle near-zero w component in qform
NIfTI: handle near-zero w component in qform

As discussed in #1318

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