fix execution of plugin in Gazebo

[cyborg-x1]

had some issues with executing the plugin, I first cloned the electromagnet branch of larics, after I got the plugin running I decided to go back to your master cause that feature seems to be not finished yet completely. Maybe I do some more changes later, but maybe you want those here - Back to the issue: The plugin was completely not executed by Gazebo on my system - no error no warning, just nothing. After I copied stuff from my plugin I once wrote, it started working. I do not know what exactly did the trick but you mainly missed the catkin_package command in the CMakeLists.txt - with that in place you would also have noticed that you missed the geometry_msgs dependency in the package.xml which I also added. I mainly copied that stuff from gazebo_plugins when I did my plugin.

[azeey]

@cyborg-x1, thanks for the pull request. I'm also not sure why things didn't work for you. One way to debug gazebo is to run rosrun gazebo_ros gazebo --verbose dipole_magnet.world. Note, however, that when interrupting this process with 'Ctrl-c', it doesn't kill gzserver so you'll have to run pkill gzserver afterwards.

Looking through your changes, I see that you have added a dependency on gazebo_plugins. This package contains other gazebo plugins none of which our package depends on, so I believe gazebo_plugins is not necessary. I have tested that on my PC without that and it works fine. If you could remove that and test it, I will go ahead and accept your pull request.