Passing the MorphableModel by const& in shape-fitting

It doesn't make a real performance difference (since all matrices are smartpointers) but it's better practice.

include/eos/fitting/linear_shape_fitting.hpp

@@ -57,7 +57,7 @@ namespace eos {
  * @param[in] model_standard_deviation The standard deviation of the 3D vertex points in the 3D model, projected to 2D (so the value is in pixels).
  * @return The estimated shape-coefficients (alphas).
  */
-inline std::vector<float> fit_shape_to_landmarks_linear(morphablemodel::MorphableModel morphable_model, cv::Mat affine_camera_matrix, std::vector<cv::Vec2f> landmarks, std::vector<int> vertex_ids, cv::Mat base_face=cv::Mat(), float lambda=3.0f, boost::optional<int> num_coefficients_to_fit=boost::optional<int>(), boost::optional<float> detector_standard_deviation=boost::optional<float>(), boost::optional<float> model_standard_deviation=boost::optional<float>())
+inline std::vector<float> fit_shape_to_landmarks_linear(const morphablemodel::MorphableModel& morphable_model, cv::Mat affine_camera_matrix, std::vector<cv::Vec2f> landmarks, std::vector<int> vertex_ids, cv::Mat base_face=cv::Mat(), float lambda=3.0f, boost::optional<int> num_coefficients_to_fit=boost::optional<int>(), boost::optional<float> detector_standard_deviation=boost::optional<float>(), boost::optional<float> model_standard_deviation=boost::optional<float>())
 {
 	using cv::Mat;
 	assert(landmarks.size() == vertex_ids.size());