add numerical recipes library

lib/nr/cpp/recipes/ratval.cpp

@@ -0,0 +1,11 @@
+#include "nr.h"
+
+DP NR::ratval(const DP x, Vec_I_DP &cof, const int mm, const int kk)
+{
+	int j;
+	DP sumd,sumn;
+
+	for (sumn=cof[mm],j=mm-1;j>=0;j--) sumn=sumn*x+cof[j];
+	for (sumd=0.0,j=mm+kk;j>mm;j--) sumd=(sumd+cof[j])*x;
+	return sumn/(1.0+sumd);
+}