add numerical recipes library

lib/nr/cpp/recipes/stoerm.cpp

@@ -0,0 +1,33 @@
+#include "nr.h"
+
+void NR::stoerm(Vec_I_DP &y, Vec_I_DP &d2y, const DP xs,
+	const DP htot, const int nstep, Vec_O_DP &yout,
+	void derivs(const DP, Vec_I_DP &, Vec_O_DP &))
+{
+	int i,nn,n,neqns;
+	DP h,h2,halfh,x;
+
+	int nv=y.size();
+	Vec_DP ytemp(nv);
+	h=htot/nstep;
+	halfh=0.5*h;
+	neqns=nv/2;
+	for (i=0;i<neqns;i++) {
+		n=neqns+i;
+		ytemp[i]=y[i]+(ytemp[n]=h*(y[n]+halfh*d2y[i]));
+	}
+	x=xs+h;
+	derivs(x,ytemp,yout);
+	h2=h*h;
+	for (nn=1;nn<nstep;nn++) {
+		for (i=0;i<neqns;i++)
+			ytemp[i] += (ytemp[neqns+i] += h2*yout[i]);
+		x += h;
+		derivs(x,ytemp,yout);
+	}
+	for (i=0;i<neqns;i++) {
+		n=neqns+i;
+		yout[n]=ytemp[n]/h+halfh*yout[i];
+		yout[i]=ytemp[i];
+	}
+}