[Samples] Improve output of 'combustor' sample C++ program

Switch to integrating using 'advance' instead of step to make output comparable,
independent of small changes in the integrator. Output only selected species and
trim the integration period to reduce the size of the output file (from 3 MB to
25 kB).

samples/cxx/combustor/combustor.cpp

@@ -83,7 +83,7 @@ void runexample()
     // time-dependent igniter mass flow rate.
     double A = 0.1;
     double FWHM = 0.2;
-    double t0 = 1.0;
+    double t0 = 0.5;
     Gaussian igniter_mdot(A, t0, FWHM);
 
     MassFlowController m3;
@@ -102,22 +102,32 @@ void runexample()
 
     // take single steps to 6 s, writing the results to a CSV file
     // for later plotting.
-    double tfinal = 6.0;
+    double tfinal = 1.0;
     double tnow = 0.0;
     double tres;
     int k;
 
     std::ofstream f("combustor_cxx.csv");
+    std::vector<size_t> k_out;
+    k_out.push_back(gas.speciesIndex("CH4"));
+    k_out.push_back(gas.speciesIndex("O2"));
+    k_out.push_back(gas.speciesIndex("CO2"));
+    k_out.push_back(gas.speciesIndex("H2O"));
+    k_out.push_back(gas.speciesIndex("CO"));
+    k_out.push_back(gas.speciesIndex("OH"));
+    k_out.push_back(gas.speciesIndex("H"));
+    k_out.push_back(gas.speciesIndex("C2H6"));
 
     while (tnow < tfinal) {
-        tnow = sim.step(tfinal);
+        tnow += 0.005;
+        sim.advance(tnow);
         tres = combustor.mass()/v.massFlowRate();
         f << tnow << ", "
           << combustor.temperature() << ", "
           << tres << ", ";
         ThermoPhase& c = combustor.contents();
-        for (k = 0; k < nsp; k++) {
-            f << c.moleFraction(k) << ", ";
+        for (size_t i = 0; i < k_out.size(); i++) {
+            f << c.moleFraction(k_out[i]) << ", ";
         }
         f << std::endl;
     }