add numerical recipes library

lib/nr/cpp/recipes/expdev.cpp

@@ -0,0 +1,13 @@
+#include <cmath>
+#include "nr.h"
+using namespace std;
+
+DP NR::expdev(int &idum)
+{
+	DP dum;
+
+	do
+		dum=ran1(idum);
+	while (dum == 0.0);
+	return -log(dum);
+}