add numerical recipes library

lib/nr/k_and_r/recipes/factln.c

@@ -0,0 +1,13 @@
+
+float factln(n)
+int n;
+{
+	float gammln();
+	void nrerror();
+	static float a[101];
+
+	if (n < 0) nrerror("Negative factorial in routine factln");
+	if (n <= 1) return 0.0;
+	if (n <= 100) return a[n] ? a[n] : (a[n]=gammln(n+1.0));
+	else return gammln(n+1.0);
+}