add numerical recipes library

lib/nr/k_and_r/recipes/pythag.c

@@ -0,0 +1,13 @@
+
+#include <math.h>
+#include "nrutil.h"
+
+float pythag(a,b)
+float a,b;
+{
+	float 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)));
+}