purgeStaleObservations

将timestamp较早的点都移除出observation_list_ 类型std::list<Observation>
唯一调用的地方是bufferCloud

if (!observation_list_.empty() )
{
    list<Observation>::iterator obs_it = observation_list_.begin();
    // if keeping observations for no time, only keep one observation

    // observation_keep_time_ 就是参数 observation_persistence
    if (observation_keep_time_ == ros::Duration(0.0))
    {
		  observation_list_.erase(++obs_it, observation_list_.end());
		  return;
    }
    // or loop through the observations to see which ones are stale
    for ( obs_it = observation_list_.begin(); obs_it != observation_list_.end(); 
            ++obs_it )
    {
		Observation& obs = *obs_it;
		// if observation is out of date, remove it and those follow from the list
		// kinetic: ros::Duration time_diff = last_updated_ - pcl_conversions::fromPCL(obs.cloud_->header).stamp;
		if ((last_updated_ - obs.cloud_->header.stamp) > observation_keep_time_)
		{
  			observation_list_.erase(obs_it, observation_list_.end());
  			return;
		}
    }
}