Add const handle concept.

Circulator/doc/Circulator/Concepts/ConstHandle.h

@@ -0,0 +1,34 @@
+
+/*!
+\ingroup PkgHandlesAndCirculatorsConcepts
+\cgalConcept
+
+A constant handle. Refer to the `Handle` concept for more details.
+
+\cgalRefines Descriptor
+
+\cgalHasModel const T* (const pointers)
+
+\sa `Handle` 
+
+*/
+class ConstHandle {
+public:
+
+/// \name Dereference 
+/// @{
+
+/*!
+returns the object pointed to. 
+*/ 
+const value_type& operator*(); 
+
+/*!
+returns a pointer to the object pointed to. 
+*/ 
+const value_type* operator->(); 
+
+/// @}
+
+}; /* end Handle */
+