add numerical recipes library

lib/nr/cpp/recipes/bessi0.cpp

@@ -0,0 +1,22 @@
+#include <cmath>
+#include "nr.h"
+using namespace std;
+
+DP NR::bessi0(const DP x)
+{
+	DP ax,ans,y;
+
+	if ((ax=fabs(x)) < 3.75) {
+		y=x/3.75;
+		y*=y;
+		ans=1.0+y*(3.5156229+y*(3.0899424+y*(1.2067492
+			+y*(0.2659732+y*(0.360768e-1+y*0.45813e-2)))));
+	} else {
+		y=3.75/ax;
+		ans=(exp(ax)/sqrt(ax))*(0.39894228+y*(0.1328592e-1
+			+y*(0.225319e-2+y*(-0.157565e-2+y*(0.916281e-2
+			+y*(-0.2057706e-1+y*(0.2635537e-1+y*(-0.1647633e-1
+			+y*0.392377e-2))))))));
+	}
+	return ans;
+}