dwifslpreproc: Refine slspec modification
dwifslpreproc: Refine slspec modification

This is necessary to facilitate the case where FSL eddy slice-to-volume correction is used, EddyQC is invoked, the number of slices is an odd number, and the slice encoding direction has negative polarity.

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