message: Use eos::cpp17::optional in Matlab bindings instead of std::
diff: #include "eos/morphablemodel/Blendshape.hpp"
  #include "eos/morphablemodel/EdgeTopology.hpp"
  #include "eos/morphablemodel/MorphableModel.hpp"
  +#include "eos/cpp17/optional.hpp"
  #include "mexplus_eigen.hpp"
  #include "mexplus_eos_types.hpp"
  #include "mex.h"
  -#include <optional>
  #include <string>
  void mexFunction(int nlhs, mxArray* plhs[], int nrhs, const mxArray* prhs[])
  @@ -91,8 +91,8 @@ void mexFunction(int nlhs, mxArray* plhs[], int nrhs, const mxArray* prhs[])
  const auto edge_topology = morphablemodel::load_edge_topology(edgetopo_file);
  const auto contour_landmarks = fitting::ContourLandmarks::load(contour_lms_file);
  const auto model_contour = fitting::ModelContour::load(model_cnt_file);
  - const std::optional<int> num_shape_coefficients_to_fit =
  - num_shape_coeffs == -1 ? std::nullopt : std::optional<int>(num_shape_coeffs);
  + const cpp17::optional<int> num_shape_coefficients_to_fit =
  + num_shape_coeffs == -1 ? cpp17::nullopt : cpp17::optional<int>(num_shape_coeffs);
  // Now do the actual fitting:
  core::Mesh mesh;
  