add numerical recipes library

lib/nr/cpp/recipes/beta.cpp

@@ -0,0 +1,8 @@
+#include <cmath>
+#include "nr.h"
+using namespace std;
+
+DP NR::beta(const DP z, const DP w)
+{
+	return exp(gammln(z)+gammln(w)-gammln(z+w));
+}