message: Make GetAgents virtual so it can be used in the observed world
diff: @@ -111,7 +111,7 @@ class World : public commons::BaseType {
  double GetWorldTime() const { return world_time_; }
  void SetWorldTime(const double& world_time) { world_time_ = world_time; }
  world::map::MapInterfacePtr GetMap() const { return map_; }
  - AgentMap GetAgents() const { return agents_; }
  + virtual AgentMap GetAgents() const { return agents_; }
  AgentMap GetValidAgents() const;
  AgentPtr GetAgent(AgentId id) const {
  auto agent_it = agents_.find(id);
  