add an overload of CGAL::to_double for Lazy_alpha_nt_3

Alpha_shapes_3/doc/Alpha_shapes_3/CGAL/Alpha_shape_3.h

@@ -73,6 +73,7 @@ resorting to exact arithmetic). Access to the interval containing the exact valu
 with `Protected=true`. Access to the exact value is provided through the function 
 `FT::Exact_nt exact() const` where `FT::Exact_nt` depends on the configuration of %CGAL 
 (it is `Gmpq` if `gmp` is available and `Quotient<CGAL::MP_Float>` otherwise).
+An overload for the function `double to_double(FT)` is also available.
 It must be noted that an object of type `FT` is valid as long as the alpha shapes class that creates 
 it is valid and has not been modified. 
 For convenience, classical comparison operators are provided for the type `FT`. 

Alpha_shapes_3/include/CGAL/internal/Lazy_alpha_nt_3.h

@@ -337,6 +337,12 @@ struct Alpha_nt_selector_3:
 
 } //namespace internal
 
+template<class Input_traits, class Kernel_input, bool mode, class Weighted_tag>
+double to_double(const internal::Lazy_alpha_nt_3<Input_traits, Kernel_input, mode, Weighted_tag>& a)
+{
+  return to_double(a.approx());
+}
+
 } //namespace CGAL
 
 #endif //CGAL_INTERNAL_LAZY_ALPHA_NT_3_H

Alpha_shapes_3/test/Alpha_shapes_3/include/CGAL/_test_cls_alpha_shape_3.h

@@ -111,6 +111,7 @@ _test_cls_alpha_shape_3()
     a1.set_alpha(*alpha_it);
     if (verbose) {
       std::cerr << std::endl;
+      CGAL::to_double(* alpha_it);
       std::cerr << "alpha value " << * alpha_it << std::endl;
     }
     count_faces(a1, verbose);