add numerical recipes library

lib/nr/cpp/recipes/rtnewt.cpp

@@ -0,0 +1,23 @@
+#include <cmath>
+#include "nr.h"
+using namespace std;
+
+DP NR::rtnewt(void funcd(const DP, DP &, DP &), const DP x1, const DP x2,
+	const DP xacc)
+{
+	const int JMAX=20;
+	int j;
+	DP df,dx,f,rtn;
+
+	rtn=0.5*(x1+x2);
+	for (j=0;j<JMAX;j++) {
+		funcd(rtn,f,df);
+		dx=f/df;
+		rtn -= dx;
+		if ((x1-rtn)*(rtn-x2) < 0.0)
+			nrerror("Jumped out of brackets in rtnewt");
+		if (fabs(dx) < xacc) return rtn;
+	}
+	nrerror("Maximum number of iterations exceeded in rtnewt");
+	return 0.0;
+}