add numerical recipes library

lib/nr/cpp/recipes/sprsax.cpp

@@ -0,0 +1,16 @@
+#include "nr.h"
+
+void NR::sprsax(Vec_I_DP &sa, Vec_I_INT &ija, Vec_I_DP &x, Vec_O_DP &b)
+{
+	int i,k;
+
+	int n=x.size();
+	if (ija[0] != n+1)
+		nrerror("sprsax: mismatched vector and matrix");
+	for (i=0;i<n;i++) {
+		b[i]=sa[i]*x[i];
+		for (k=ija[i];k<ija[i+1];k++) {
+			b[i] += sa[k]*x[ija[k]];
+		}
+	}
+}