message: Core: fixed timings in Optimizer.
  For some reason, the bug did not appear on Windows.
diff: @@ -106,7 +106,7 @@ namespace Engine
  {
  ++step;
  - t_last = t_current;
  + t_current = system_clock::now();
  maxforce_stream.str(std::string());
  maxforce_stream.clear();
  @@ -126,7 +126,7 @@ namespace Engine
  this->method->Save_Current(this->starttime, iteration, false, false);
  - //output_strings[step - 1] = IO::Spins_to_String(c->images[0].get());
  + t_last = t_current;
  }
  // Unlock Systems
  