add numerical recipes library

lib/nr/cpp/recipes/pythag.cpp

@@ -0,0 +1,13 @@
+#include <cmath>
+#include "nr.h"
+using namespace std;
+
+DP NR::pythag(const DP a, const DP b)
+{
+	DP absa,absb;
+
+	absa=fabs(a);
+	absb=fabs(b);
+	if (absa > absb) return absa*sqrt(1.0+SQR(absb/absa));
+	else return (absb == 0.0 ? 0.0 : absb*sqrt(1.0+SQR(absa/absb)));
+}