Fix conversion warnings

Classification/include/CGAL/Classification/OpenCV_random_forest_classifier.h

@@ -158,14 +158,14 @@ public:
     
     std::size_t nb_samples = 0;
     for (std::size_t i = 0; i < ground_truth.size(); ++ i)
-      if (ground_truth[i] != -1)
+      if (int(ground_truth[i]) != -1)
         ++ nb_samples;
 
     cv::Mat training_features (int(nb_samples), int(m_features.size()), CV_32FC1);
     cv::Mat training_labels (int(nb_samples), 1, CV_32FC1);
 
     for (std::size_t i = 0, index = 0; i < ground_truth.size(); ++ i)
-      if (ground_truth[i] != -1)
+      if (int(ground_truth[i]) != -1)
       {
         for (std::size_t f = 0; f < m_features.size(); ++ f)
           training_features.at<float>(int(index), int(f)) = m_features[f]->value(i);

Classification/include/CGAL/Classification/Sum_of_weighted_features_classifier.h

@@ -311,7 +311,7 @@ public:
     std::vector<std::vector<std::size_t> > training_sets (m_labels.size());
     std::size_t nb_tot = 0;
     for (std::size_t i = 0; i < ground_truth.size(); ++ i)
-      if (ground_truth[i] != -1)
+      if (int(ground_truth[i]) != -1)
       {
         training_sets[std::size_t(ground_truth[i])].push_back (i);
         ++ nb_tot;

Polyhedron/demo/Polyhedron/Plugins/Classification/Surface_mesh_item_classification.cpp

@@ -202,9 +202,9 @@ void Surface_mesh_item_classification::train
   BOOST_FOREACH(face_descriptor fd, faces(*(m_mesh->polyhedron())))
   {
     training[fd] = m_training[fd];
-    if (training[fd] != -1)
+    if (training[fd] != std::size_t(-1))
     {
-      nb_label[std::size_t(training[fd])] ++;
+      nb_label[training[fd]] ++;
       ++ nb_total;
     }
   }