Predefined directions now generated using r^2 electrostatic repulsion
Predefined directions now generated using r^2 electrostatic repulsion

Previous predefined directions were generated using a r^128 repulsion
term, similar to Papadakis et al, MRI 18, 671–679 (2000), on the basis
that this produced a wider average nearest-neighbour angle. We recently
observed that this leads to suboptimal SH transforms compared to using
the more widely used r^2 term. For this reason, dirgen has been changed
to use an r^2 repulsion term by default (as per commit ed9c8f5).

The present commit propagates this change to the predefined direction
sets included in MRtrix3. This should minimises problems such as those
highlighted for dwi2fod in issue #201.

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