responsemean: Change overall RF scaling
responsemean: Change overall RF scaling

Ensures that the l=0 terms of the resulting RF come out to be comparable to the averages of the input l=0 terms (constrained by the fact that a single scalar multiplier is used per subject across all shells).

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