Set initial maps and pose

• Update config file to add new features:
  • Allow to load initial keypoints maps at node startup
  • Allow to freeze these initial maps to avoid updating them
  • Allow to set initial SLAM pose at startup
  • Rename maps_saving/pcd_format to maps/export_pcd_format
• Add a subscriber to 'set_slam_pose' topic (of type geometry_msgs/PoseWithCovarianceStamped) to set SLAM pose from external guess. This allows to set SLAM pose from RViz using the 'set initial 2D pose', or by publishing a full 3D pose using an other localization node.

@julia.sanchez @nick.laurenson PTAL