Added several comments at Skeletonization 3D and RenderSkeletonization
Added option to show and hide skeleton, now hidding cameras does not hide the skeleton.
Added quad to show the the 2D skeletonization in the back of the 3D scene.
It works when loading images, but with data from Skeletonization class
the dog is upside down and repeated 6 times
Fix texture coordenates so the skeleton is no longer upside-down.
It is still repeated 6 times
Fixed repetead skeleton in texture
Added code to show 2D skeletons of each camera
Moved code to display skeleton in 2D and 3D to methods
inside RenderSkeleton.

Code is cleaner now
Minor, comments added
Added green color to 2D skeleton projections
Changed method name from get_points_for_camera to get_simple_3d_projection
since is much more descriptive.
Slekeletonization2D now uses a pointer to a camera instead of a pointer
to some frames. That way it can use the camera getter methods.
Deleted method to access all frames directly in RGBDCamera.

More encapsulation.
Added several const modifiers
More const additions
More const adding
Fix unused variables and not initilized variables compiler warnings.
Changed initialization order in MultiCamviewer, now there are no more
wreorder warnings
Modified pointer to camera_array, now the vector is copied.
Skeletonization3D
Added merge_treshold.
Moved back skel_array atribute to private.
Added method get_2D_frame.
Minor change on merge method