Replaced a few more cpp17::optional with std::optional

I missed these in the previous commits.

examples/fit-model.cpp

@@ -28,7 +28,6 @@
 #include "eos/morphablemodel/MorphableModel.hpp"
 #include "eos/render/opencv/draw_utils.hpp"
 #include "eos/render/texture_extraction.hpp"
-#include "eos/cpp17/optional.hpp"
 
 #include "Eigen/Core"
 
@@ -142,7 +141,7 @@ int main(int argc, char* argv[])
         morphablemodel::load_blendshapes(blendshapesfile.string());
 
     morphablemodel::MorphableModel morphable_model_with_expressions(
-        morphable_model.get_shape_model(), blendshapes, morphable_model.get_color_model(), cpp17::nullopt,
+        morphable_model.get_shape_model(), blendshapes, morphable_model.get_color_model(), std::nullopt,
         morphable_model.get_texture_coordinates());
 
     // These two are used to fit the front-facing contour to the ibug contour landmarks:
@@ -167,7 +166,7 @@ int main(int argc, char* argv[])
     fitting::RenderingParameters rendering_params;
     std::tie(mesh, rendering_params) = fitting::fit_shape_and_pose(
         morphable_model_with_expressions, landmarks, landmark_mapper, image.cols, image.rows, edge_topology,
-        ibug_contour, model_contour, 5, cpp17::nullopt, 30.0f);
+        ibug_contour, model_contour, 5, std::nullopt, 30.0f);
 
     // The 3D head pose can be recovered as follows - the function returns an Eigen::Vector3f with yaw, pitch,
     // and roll angles:

include/eos/core/read_obj.hpp

@@ -23,7 +23,6 @@
 #define EOS_READ_OBJ_HPP
 
 #include "eos/core/Mesh.hpp"
-#include "eos/cpp17/optional.hpp"
 
 #include "Eigen/Core"
 
@@ -33,6 +32,7 @@
 #include <tuple>
 #include <utility>
 #include <vector>
+#include <optional>
 
 namespace eos {
 namespace core {
@@ -88,7 +88,7 @@ void tokenize(const std::string& str, ContainerType& tokens, const std::string&
  * Another obj parser we can check: https://github.com/qnzhou/PyMesh/blob/master/src/IO/OBJParser.cpp (and
  * same file with .h)
  */
-inline std::pair<Eigen::Vector3f, cpp17::optional<Eigen::Vector3f>> parse_vertex(const std::string& line)
+inline std::pair<Eigen::Vector3f, std::optional<Eigen::Vector3f>> parse_vertex(const std::string& line)
 {
     std::vector<std::string> tokens;
     tokenize(line, tokens, " ", true); // compress multiple (and leading/trailing) whitespaces
@@ -99,7 +99,7 @@ inline std::pair<Eigen::Vector3f, cpp17::optional<Eigen::Vector3f>> parse_vertex
             "or 6 ('x y z r g b') numbers.");
     }
     const Eigen::Vector3f vertex(std::stof(tokens[0]), std::stof(tokens[1]), std::stof(tokens[2]));
-    cpp17::optional<Eigen::Vector3f> vertex_color;
+    std::optional<Eigen::Vector3f> vertex_color;
     if (tokens.size() == 6)
     {
         vertex_color = Eigen::Vector3f(std::stof(tokens[3]), std::stof(tokens[4]), std::stof(tokens[5]));

include/eos/morphablemodel/MorphableModel.hpp

@@ -140,7 +140,7 @@ class MorphableModel
     /**
      * Returns the shape expression model, if this Morphable Model has one.
      *
-     * Returns an empty cpp17::optional if the Morphable Model does not have a separate expression
+     * Returns an empty std::optional if the Morphable Model does not have a separate expression
      * model (check with MorphableModel::has_separate_expression_model()).
      * If it does have an expression model, an std::variant<PcaModel, Blendshapes> is returned -
      * that is, either a PcaModel (if it is an expression PCA model), or Blendshapes.