Add possibiblity to access the densities function

- sometimes one needs the ability to access data from the densities function -
  if this is the case, then one capture it by simply using
  `multi_channel_point2`.

include/hep/mc/multi_channel.hpp

@@ -27,6 +27,7 @@
 #include <cstddef>
 #include <limits>
 #include <random>
+#include <type_traits>
 #include <utility>
 #include <vector>
 
@@ -144,8 +145,9 @@ inline multi_channel_result<T> multi_channel_iteration(
 			total_density += channel_weights[j] * channel_densities[j];
 		}
 
-		multi_channel_point<T> const point(total_calls, random_numbers,
-			coordinates, channel, total_density);
+		multi_channel_point2<T, typename std::remove_reference<D>::type> const
+			point(total_calls, random_numbers, coordinates, channel,
+			total_density, densities);
 
 		T const value = function(point) * point.weight;
 

include/hep/mc/multi_channel_point.hpp

@@ -56,6 +56,33 @@ struct multi_channel_point : public mc_point<T>
 	std::vector<T> const& coordinates;
 };
 
+/// Point in the unit-hypercube for multi-channel Monte Carlo integration. This
+/// type also captures the density functions that are used to generate
+/// `coordinates`.
+template <typename T, typename D>
+struct multi_channel_point2 : public multi_channel_point<T>
+{
+	/// Constructor.
+	multi_channel_point2(
+		std::size_t calls,
+		std::vector<T> const& point,
+		std::vector<T> const& coordinates,
+		std::size_t channel,
+		T total_density,
+		D const& density_function
+	)
+		: multi_channel_point<T>(calls, point, coordinates, channel,
+			total_density)
+		, density_function(density_function)
+	{
+	}
+
+	/// The density function that constructed this point. See
+	/// \ref multi_channel_iteration for reference of the signature of this
+	/// function.
+	D const& density_function;
+};
+
 /// @}
 
 }