add numerical recipes library

lib/nr/cpp/recipes/quadvl.cpp

@@ -0,0 +1,15 @@
+#include "nr.h"
+
+void NR::quadvl(const DP x, const DP y, DP &fa, DP &fb, DP &fc, DP &fd)
+{
+	DP qa,qb,qc,qd;
+
+	qa=MIN(2.0,MAX(0.0,1.0-x));
+	qb=MIN(2.0,MAX(0.0,1.0-y));
+	qc=MIN(2.0,MAX(0.0,x+1.0));
+	qd=MIN(2.0,MAX(0.0,y+1.0));
+	fa=0.25*qa*qb;
+	fb=0.25*qb*qc;
+	fc=0.25*qc*qd;
+	fd=0.25*qd*qa;
+}