add numerical recipes library

lib/nr/cpp/recipes/gammln.cpp

@@ -0,0 +1,19 @@
+#include <cmath>
+#include "nr.h"
+using namespace std;
+
+DP NR::gammln(const DP xx)
+{
+	int j;
+	DP x,y,tmp,ser;
+	static const DP cof[6]={76.18009172947146,-86.50532032941677,
+		24.01409824083091,-1.231739572450155,0.1208650973866179e-2,
+		-0.5395239384953e-5};
+
+	y=x=xx;
+	tmp=x+5.5;
+	tmp -= (x+0.5)*log(tmp);
+	ser=1.000000000190015;
+	for (j=0;j<6;j++) ser += cof[j]/++y;
+	return -tmp+log(2.5066282746310005*ser/x);
+}