message: Core: factor ~1.5 speedup of LLG.
  The Spin_System::UpdateEnergy function was unnecessarily calculating the Energies twice, creating an unnecessarily large performance impact.
diff: @@ -104,7 +104,9 @@ namespace Data
  void Spin_System::UpdateEnergy()
  {
  this->E_array = this->hamiltonian->Energy_Contributions(*this->spins);
  - this->E = this->hamiltonian->Energy(*this->spins);
  + scalar sum = 0;
  + for (auto E : E_array) sum += E.second;
  + this->E = sum;
  }
  void Spin_System::UpdateEffectiveField()
  