add numerical recipes library

lib/nr/k_and_r/recipes/ellf.c

@@ -0,0 +1,13 @@
+
+#include <math.h>
+#include "nrutil.h"
+
+float ellf(phi,ak)
+float ak,phi;
+{
+	float rf();
+	float s;
+
+	s=sin(phi);
+	return s*rf(SQR(cos(phi)),(1.0-s*ak)*(1.0+s*ak),1.0);
+}