made secondary particle bank particle owned instead of thread private

include/openmc/particle.h

@@ -294,6 +294,9 @@ public:
   // Current PRNG state
   uint64_t seeds_[N_STREAMS]; // current seeds
   int      stream_;           // current RNG stream
+  
+  // Secondary bank
+  std::vector<Particle::Bank> secondary_bank_;
 };
 
 } // namespace openmc