Add initial pose subscriber

[nahueespinosa]

This patch adds an initial pose subscriber to beluga_amcl that logs the received pose.

Related to #41.

[glpuga]

LGTM.

If we are not going to fully implement this functionality soon, I'd add a very visible WARNING log at the end of the callback to announce that for the time being the new pose is only logged, but not really used.

If we are going to address this in the next few days, then there's no need to add any log.

[olmerg]

I was making the same solution in branch initial_pose_ol, but after I transform to eigen I cannot update the estimated particle filter pose. I test with

  particle_filter_->update_motion( out); 
  particle_filter_->importance_sample();
  particle_filter_->resample();

It take a lot of time(steps) to update the estimated pose (I only rotate the robot).
y
I will follow the solution to understand Beluga :)

Just to take account about this issue nav2_amcl also support add a initialpose parameter:

  get_parameter("set_initial_pose", set_initial_pose_);
  get_parameter("initial_pose.x", initial_pose_x_);
  get_parameter("initial_pose.y", initial_pose_y_);
  get_parameter("initial_pose.z", initial_pose_z_);
  get_parameter("initial_pose.yaw", initial_pose_yaw_);

[nahueespinosa]

I'd add a very visible WARNING log at the end of the callback to announce that for the time being the new pose is only logged, but not really used.

@glpuga Sounds good! Done in 7519731.

@olmerg Right, there is no way to set an initial pose for the particle filter yet. About the parameters, that's correct, it's mentioned in #41.