add numerical recipes library

lib/nr/cpp/recipes/igray.cpp

@@ -0,0 +1,17 @@
+#include "nr.h"
+
+unsigned long NR::igray(const unsigned long n, const int is)
+{
+	int ish;
+	unsigned long ans,idiv;
+
+	if (is >= 0)
+		return n ^ (n >> 1);
+	ish=1;
+	ans=n;
+	for (;;) {
+		ans ^= (idiv=ans >> ish);
+		if (idiv <= 1 || ish == 16) return ans;
+		ish <<= 1;
+	}
+}