message: Core: moved mu_B back to the gradient in DDI interaction.
  The gradient was scaled incorrectly, as a factor mu_B was missing in the gradient, which was placed into the energy.
diff: @@ -387,7 +387,7 @@ namespace Engine
  {
  for(int idx = blockIdx.x * blockDim.x + threadIdx.x; idx < nos; idx += blockDim.x * gridDim.x)
  {
  - Energy[idx] += 0.5 * mu_s[idx] * C::mu_B * spins[idx].dot(gradients[idx]);
  + Energy[idx] += 0.5 * mu_s[idx] * spins[idx].dot(gradients[idx]);
  }
  }
  void Hamiltonian_Heisenberg::E_DDI_FFT(const vectorfield & spins, scalarfield & Energy)
  @@ -988,7 +988,7 @@ namespace Engine
  void Hamiltonian_Heisenberg::FFT_Dipole_Mats(std::array<int, 3> pb_images)
  {
  // Prefactor of ddi interaction
  - scalar mult = 2 * C::mu_0 * C::mu_B / ( 4*C::Pi * 1e-30 );
  + scalar mult = 2 * C::mu_0 * C::mu_B * C::mu_B / ( 4*C::Pi * 1e-30 );
  // Size of original geometry
  int Na = geometry->n_cells[0];
  