added method to know if a spatial ubdexing structure is empty or not

vcg/space/index/grid_static_ptr.h

@@ -132,6 +132,7 @@ namespace vcg {
 
 
 
+    bool Empty() const {return links.empty();}
 
 		/// Date le coordinate di un grid point (corner minx,miy,minz) ritorna le celle che condividono
 		/// l'edge cell che parte dal grid point in direzione axis

vcg/space/index/spatial_hashing.h

@@ -115,6 +115,10 @@ namespace vcg{
 		void operator ++() {t++;}
 	};
 	
+  inline bool Empty() const
+  {
+    return hash_table.empty();
+  }
 
     size_t CellSize(const Point3i &cell)
     {