add numerical recipes library

lib/nr/cpp/recipes/bessk0.cpp

@@ -0,0 +1,21 @@
+#include <cmath>
+#include "nr.h"
+using namespace std;
+
+DP NR::bessk0(const DP x)
+{
+	DP y,ans;
+
+	if (x <= 2.0) {
+		y=x*x/4.0;
+		ans=(-log(x/2.0)*bessi0(x))+(-0.57721566+y*(0.42278420
+			+y*(0.23069756+y*(0.3488590e-1+y*(0.262698e-2
+			+y*(0.10750e-3+y*0.74e-5))))));
+	} else {
+		y=2.0/x;
+		ans=(exp(-x)/sqrt(x))*(1.25331414+y*(-0.7832358e-1
+			+y*(0.2189568e-1+y*(-0.1062446e-1+y*(0.587872e-2
+			+y*(-0.251540e-2+y*0.53208e-3))))));
+	}
+	return ans;
+}