add numerical recipes library

lib/nr/cpp/recipes/julday.cpp

@@ -0,0 +1,24 @@
+#include <cmath>
+#include "nr.h"
+using namespace std;
+
+int NR::julday(const int mm, const int id, const int iyyy)
+{
+	const int IGREG=15+31*(10+12*1582);
+	int ja,jul,jy=iyyy,jm;
+
+	if (jy == 0) nrerror("julday: there is no year zero.");
+	if (jy < 0) ++jy;
+	if (mm > 2) {
+		jm=mm+1;
+	} else {
+		--jy;
+		jm=mm+13;
+	}
+	jul = int(floor(365.25*jy)+floor(30.6001*jm)+id+1720995);
+	if (id+31*(mm+12*iyyy) >= IGREG) {
+		ja=int(0.01*jy);
+		jul += 2-ja+int(0.25*ja);
+	}
+	return jul;
+}