Write a wrapper for the optional variable needed for AutoDiff to avoid duplication of code

[tarun-mitruka]

if (dx) {
  for (auto i = 0U; i < disp.size(); ++i)
    for (auto k2 = 0U; k2 < myDim; ++k2)
      disp[i][k2] = dx.value()[i * myDim + k2]
                    + d[this->localView().index(this->localView().tree().child(k2).localIndex(i))[0]];
} else
  for (auto i = 0U; i < disp.size(); ++i)
    for (auto k2 = 0U; k2 < myDim; ++k2)
      disp[i][k2] = d[this->localView().index(this->localView().tree().child(k2).localIndex(i))[0]];