RNG seed as environmental variable

forward_ions.cxx

@@ -13,7 +13,7 @@ void forward_ions(){
   WriterAscii hepmc_output("data/forward_ions.hepmc");
   int        events_parsed = 0;
   GenEvent   evt(Units::GEV, Units::MM);
-  std::mt19937 gen(1337);
+  std::mt19937 gen( std::stoi(std::getenv("SEED")) );
   std::uniform_real_distribution<double> phi_distr(0,6.2831853072);
   std::uniform_real_distribution<double> theta_distr(0,0.01745329252);