mrdegibbs: use full precision Math::pi
mrdegibbs: use full precision Math::pi

This requires explicit handling of potential divide-by-zero later in the
code.

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