dirmerge: reinstate moderate bias towards unipolar model
dirmerge: reinstate moderate bias towards unipolar model

and add option to modify it (note this changes the default balance
between unipolar and bipolar slightly from what it was). Also simplify
the energy calculation using Eigen types.

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