Add a copy constructor in Cell_attribute.

This solves a bug in one of my prog, which occurs only for CMap with index.
I don't understand why the default copy constructor is not enough for
CMap with index (while it works for CMap with handle).

Combinatorial_map/include/CGAL/Cell_attribute.h

@@ -397,6 +397,11 @@ struct Init_id;
     typedef OnSplit                          On_split;
     typedef void                             Info;
 
+    // Copy constructor why this is required for CMap with index and not for CMap with handle ?
+    Cell_attribute(const Cell_attribute& other) :
+      Cell_attribute_without_info<Refs, Tag_, OnMerge, OnSplit, WithID>(other)
+    {}
+
   protected:
     /// Default constructor.
     Cell_attribute()
@@ -460,6 +465,12 @@ struct Init_id;
     bool operator!=(const Self& other) const
     { return !operator==(other); }
 
+    // Copy constructor why this is required for CMap with index and not cor CMap with handle ?
+    Cell_attribute(const Self& other) :
+      Cell_attribute_without_info<Refs, Tag_, OnMerge, OnSplit, WithID>(other),
+      Info_for_cell_attribute<Info_>(other)
+    {}
+
   protected:
     /// Default constructor.
     Cell_attribute()