add operator[] for particle_state

bfps/cpp/particles_base.hpp

@@ -48,6 +48,11 @@ class single_particle_state
 
         single_particle_state<particle_type> &operator=(const single_particle_state &src);
         single_particle_state<particle_type> &operator=(const double *src);
+
+        inline double operator[](const int i)
+        {
+            return this->data[i];
+        }
 };
 
 #endif//PARTICLES_BASE