add numerical recipes library

lib/nr/cpp/recipes/trapzd.cpp

@@ -0,0 +1,20 @@
+#include "nr.h"
+
+DP NR::trapzd(DP func(const DP), const DP a, const DP b, const int n)
+{
+	DP x,tnm,sum,del;
+	static DP s;
+	int it,j;
+
+	if (n == 1) {
+		return (s=0.5*(b-a)*(func(a)+func(b)));
+	} else {
+		for (it=1,j=1;j<n-1;j++) it <<= 1;
+		tnm=it;
+		del=(b-a)/tnm;
+		x=a+0.5*del;
+		for (sum=0.0,j=0;j<it;j++,x+=del) sum += func(x);
+		s=0.5*(s+(b-a)*sum/tnm);
+		return s;
+	}
+}