add numerical recipes library

lib/nr/cpp/recipes/rtbis.cpp

@@ -0,0 +1,22 @@
+#include <cmath>
+#include "nr.h"
+using namespace std;
+
+DP NR::rtbis(DP func(const DP), const DP x1, const DP x2, const DP xacc)
+{
+	const int JMAX=40;
+	int j;
+	DP dx,f,fmid,xmid,rtb;
+
+	f=func(x1);
+	fmid=func(x2);
+	if (f*fmid >= 0.0) nrerror("Root must be bracketed for bisection in rtbis");
+	rtb = f < 0.0 ? (dx=x2-x1,x1) : (dx=x1-x2,x2);
+	for (j=0;j<JMAX;j++) {
+		fmid=func(xmid=rtb+(dx *= 0.5));
+		if (fmid <= 0.0) rtb=xmid;
+		if (fabs(dx) < xacc || fmid == 0.0) return rtb;
+	}
+	nrerror("Too many bisections in rtbis");
+	return 0.0;
+}