dwipreproc: Catch --slspec mis-use
dwipreproc: Catch --slspec mis-use

If a user attempts to provide slice timing information to eddy via the --slspec option, instead of the slice indices for acquisition groups, then parse the contents of this file as slice timing; but give the user a slap on the wrist while doing so.
Inspired by #1360.

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