Visualize Landmark pointclouds in RVIZ

[sandeepdutta]

I have added a feature to visualize the landmarks in RVIZ as a pointcloud.
"All Landmakes" are displayed as grey, the "local landmarks" are displayed as red.
It seems to be working correctly, it would be great if the code can be reviewed and
merged into the main branch if acceptable.
The code is in my fork @ "[email protected]:sandeepdutta/stella_vslam_ros.git"
I have changed on "stella_vslam_ros.cc" ,"stall_vslam_ros.h" & "system.cc" , added a function
publish_landmarks, it is called @ 1HZ from viewer thread in "system.cc" when "viewer_string_" is "pointcloud_publish"

Please let me know if I can add anything more.
Thank you
Sandeep