add numerical recipes library

lib/nr/cpp/recipes/erfcc.cpp

@@ -0,0 +1,15 @@
+#include <cmath>
+#include "nr.h"
+using namespace std;
+
+DP NR::erfcc(const DP x)
+{
+	DP t,z,ans;
+
+	z=fabs(x);
+	t=1.0/(1.0+0.5*z);
+	ans=t*exp(-z*z-1.26551223+t*(1.00002368+t*(0.37409196+t*(0.09678418+
+		t*(-0.18628806+t*(0.27886807+t*(-1.13520398+t*(1.48851587+
+		t*(-0.82215223+t*0.17087277)))))))));
+	return (x >= 0.0 ? ans : 2.0-ans);
+}