add numerical recipes library

lib/nr/cpp/recipes/gasdev.cpp

@@ -0,0 +1,26 @@
+#include <cmath>
+#include "nr.h"
+using namespace std;
+
+DP NR::gasdev(int &idum)
+{
+	static int iset=0;
+	static DP gset;
+	DP fac,rsq,v1,v2;
+
+	if (idum < 0) iset=0;
+	if (iset == 0) {
+		do {
+			v1=2.0*ran1(idum)-1.0;
+			v2=2.0*ran1(idum)-1.0;
+			rsq=v1*v1+v2*v2;
+		} while (rsq >= 1.0 || rsq == 0.0);
+		fac=sqrt(-2.0*log(rsq)/rsq);
+		gset=v1*fac;
+		iset=1;
+		return v2*fac;
+	} else {
+		iset=0;
+		return gset;
+	}
+}