Fix example mesh_3D_gray_vtk_image (#7824)

## Summary of Changes

Fixes example mesh_3D_gray_vtk_image.
Changed : Meshing all value **under** isovalue --> Meshing all values
**over** isovalue.

## Release Management

* Affected package(s): Mesh_3
* Issue(s) solved (if any): 
* Feature/Small Feature (if any): n/a
* License and copyright ownership: no change

Mesh_3/examples/Mesh_3/mesh_3D_gray_vtk_image.cpp

@@ -36,14 +36,15 @@ typedef CGAL::Mesh_complex_3_in_triangulation_3<Tr> C3t3;
 // Criteria
 typedef CGAL::Mesh_criteria_3<Tr> Mesh_criteria;
 
-class Less {
+template <bool less_than_iso>
+class Is_interior {
   double iso;
 public:
-  Less(double iso): iso(iso) {}
+  Is_interior(double iso): iso(iso) {}
 
   template <typename T>
   int operator()(T v) const {
-    return int(v < iso);
+    return int( (v < iso) == less_than_iso);
   }
 };
 
@@ -53,7 +54,7 @@ int main(int argc, char* argv[])
 {
   // Loads image
 
-  // Usage: mesh_3D_gray_vtk_image <nii file or dicom directory> iso_level=1  facet_size=1  facet_distance=0.1  cell_size=1
+  // Usage: mesh_3D_gray_vtk_image <nii file or dicom directory> iso_level=1  facet_size=1  facet_distance=0.1  cell_size=1 interior_less_than_isolevel=true
 
   const std::string fname = (argc>1)?argv[1]:CGAL::data_file_path("images/squircle.nii");
 
@@ -62,6 +63,7 @@ int main(int argc, char* argv[])
   double fs = (argc>3)? boost::lexical_cast<double>(argv[3]): 1;
   double fd = (argc>4)? boost::lexical_cast<double>(argv[4]): 0.1;
   double cs = (argc>5)? boost::lexical_cast<double>(argv[5]): 1;
+  bool less = (argc>6)? boost::lexical_cast<bool>(argv[6]): true;
 
   fs::path path(fname);
 
@@ -108,10 +110,12 @@ int main(int argc, char* argv[])
   /// [Domain creation]
   namespace params = CGAL::parameters;
 
-  Mesh_domain domain = Mesh_domain::create_gray_image_mesh_domain
-    (image,
-     params::image_values_to_subdomain_indices(Less(iso)).
-             value_outside(iso+1));
+  Mesh_domain domain = less ? Mesh_domain::create_gray_image_mesh_domain(image,
+                                params::image_values_to_subdomain_indices(Is_interior<true>(iso)).
+                                value_outside(iso+1))
+                            : Mesh_domain::create_gray_image_mesh_domain(image,
+                                params::image_values_to_subdomain_indices(Is_interior<false>(iso)).
+                                value_outside(iso+1));
   /// [Domain creation]
 
   // Mesh criteria