add numerical recipes library

lib/nr/ansi/recipes/bico.c

@@ -0,0 +1,9 @@
+
+#include <math.h>
+
+float bico(int n, int k)
+{
+	float factln(int n);
+
+	return floor(0.5+exp(factln(n)-factln(k)-factln(n-k)));
+}