add numerical recipes library

lib/nr/cpp/recipes/zbrak.cpp

@@ -0,0 +1,22 @@
+#include "nr.h"
+
+void NR::zbrak(DP fx(const DP), const DP x1, const DP x2, const int n,
+	Vec_O_DP &xb1, Vec_O_DP &xb2, int &nroot)
+{
+	int i;
+	DP x,fp,fc,dx;
+
+	int nb=xb1.size();
+	nroot=0;
+	dx=(x2-x1)/n;
+	fp=fx(x=x1);
+	for (i=0;i<n;i++) {
+		fc=fx(x += dx);
+		if (fc*fp <= 0.0) {
+			xb1[nroot]=x-dx;
+			xb2[nroot++]=x;
+			if(nroot == nb) return;
+		}
+		fp=fc;
+	}
+}