add numerical recipes library

lib/nr/cpp/recipes/factrl.cpp

@@ -0,0 +1,18 @@
+#include <cmath>
+#include "nr.h"
+using namespace std;
+
+DP NR::factrl(const int n)
+{
+	static int ntop=4;
+	static DP a[33]={1.0,1.0,2.0,6.0,24.0};
+	int j;
+
+	if (n < 0) nrerror("Negative factorial in routine factrl");
+	if (n > 32) return exp(gammln(n+1.0));
+	while (ntop<n) {
+		j=ntop++;
+		a[ntop]=a[j]*ntop;
+	}
+	return a[n];
+}