add numerical recipes library

lib/nr/cpp/recipes/snrm.cpp

@@ -0,0 +1,22 @@
+#include <cmath>
+#include "nr.h"
+using namespace std;
+
+DP NR::snrm(Vec_I_DP &sx, const int itol)
+{
+	int i,isamax;
+	DP ans;
+
+	int n=sx.size();
+	if (itol <= 3) {
+		ans = 0.0;
+		for (i=0;i<n;i++) ans += sx[i]*sx[i];
+		return sqrt(ans);
+	} else {
+		isamax=0;
+		for (i=0;i<n;i++) {
+			if (fabs(sx[i]) > fabs(sx[isamax])) isamax=i;
+		}
+		return fabs(sx[isamax]);
+	}
+}