add numerical recipes library

lib/nr/k_and_r/recipes/ratval.c

@@ -0,0 +1,12 @@
+
+double ratval(x,cof,mm,kk)
+double cof[],x;
+int kk,mm;
+{
+	int j;
+	double 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+1;j--) sumd=(sumd+cof[j])*x;
+	return sumn/(1.0+sumd);
+}