add numerical recipes library

lib/nr/cpp/recipes/bico.cpp

@@ -0,0 +1,8 @@
+#include <cmath>
+#include "nr.h"
+using namespace std;
+
+DP NR::bico(const int n, const int k)
+{
+	return floor(0.5+exp(factln(n)-factln(k)-factln(n-k)));
+}