Electrical mating plugin

[Youssef-Khaky]

This PR is ready for review.
The associated tutorial to this PR can be found here.
This associated GitHub issue can be found here.

PR Summary:
These are tutorial pictures used for the wiki

 tutorial_pictures/collision_socket.png          
 tutorial_pictures/electrical_mating_launch.png  
 tutorial_pictures/plug_on_platform.png
 tutorial_pictures/socket_on_bop.png

These are files related to the plug and socket models

 uuv_dave/models/plug/model.config       
 uuv_dave/models/plug/model.sdf            
 uuv_dave/models/socket_box/model.config      
 uuv_dave/models/socket_box/model.sdf 
 uuv_mating/models/Electrical-Plug.dae           
 uuv_mating/models/Electrical-Socket.dae  
 uuv_mating/models/Inner-Plug.png      
 uuv_mating/models/Outer-Plug.png         

Files associated with the plugin

 uuv_mating/include/uuv_mating/uuv_mating.hh
 uuv_mating/src/uuv_mating.cc    
 uuv_dave/launch/uuv_dave_electrical_mate.launch       
 uuv_dave/worlds/uuv_dave_electrical_mating.world 
 uuv_mating/CMakeLists.txt               
 uuv_mating/package.xml               

[PenguineZhang]

Looks good to me