add numerical recipes library

lib/nr/ansi/recipes/bessk.c

@@ -0,0 +1,20 @@
+
+float bessk(int n, float x)
+{
+	float bessk0(float x);
+	float bessk1(float x);
+	void nrerror(char error_text[]);
+	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;
+}