Fix overwriting of primary writer by startOfLibrary command

examples/cascade_examples/em_shower.cpp

@@ -181,6 +181,8 @@ int main(int argc, char** argv) {
   auto sequence = make_sequence(emCascade, emContinuous, longprof, observationLevel, cut);
   // define air shower object, run simulation
   TrackingType tracking;
+  
+  output.startOfLibrary();
 
   output.startOfLibrary();