add numerical recipes library

lib/nr/k_and_r/recipes/bessk.c

@@ -0,0 +1,21 @@
+
+float bessk(n,x)
+float x;
+int n;
+{
+	float bessk0(),bessk1();
+	void nrerror();
+	int j;
+	float bk,bkm,bkp,tox;
+
+	if (n < 2) nrerror("Index n less than 2 in bessk");
+	tox=2.0/x;
+	bkm=bessk0(x);
+	bk=bessk1(x);
+	for (j=1;j<n;j++) {
+		bkp=bkm+j*tox*bk;
+		bkm=bk;
+		bk=bkp;
+	}
+	return bk;
+}