add numerical recipes library

lib/nr/k_and_r/recipes/bico.c

@@ -0,0 +1,10 @@
+
+#include <math.h>
+
+float bico(n,k)
+int k,n;
+{
+	float factln();
+
+	return floor(0.5+exp(factln(n)-factln(k)-factln(n-k)));
+}