do not kill the program when there is not freeze-out cells

Author: chunshen1987

src/iSS.cpp

@@ -127,7 +127,7 @@ int iSS::read_in_FO_surface() {
     if (FOsurf_temp.size() == 0) {
         messager << "No freeze-out fluid cell, exit now ...";
         messager.flush("Warning");
-        exit(1);
+        return(-1);
     }
 
     afterburner_type_ = freeze_out_data.get_afterburner_type();

src/readindata.cpp

@@ -601,7 +601,7 @@ void read_FOdata::read_FOsurfdat_MUSIC(std::vector<FO_surf> &surf_ptr,
     if (!surfdat.good()) {
         cout << "[Error] Surface file is not found! " << surfdat_stream.str()
              << endl;
-        exit(1);
+        return;
     }
     while (!surfdat.eof()) {
         FO_surf surf_elem;