Merge hydro into groovy

There are some users who wish to continue to use gazebo_ros_pkgs with ROS Groovy, and so to make development and bug fixes easier in the short term, I have made the two branches be clones of each other. This was accomplished by using compiler conditions ``#if`` for the different function calls for PCL 1.6 (Groovy) and PCL 1.7 (Hydro).

@wjwwood @rethink-cgindel @hsu