differential

On some platforms it is necessary to use the standard/cpu gazebo laser plugin instead of the gazebo gpu laser plugin (e.g. without dedicated gpu). Make it possible to switch between gazebo laser plugin and the gazebo gpu laser plugin and adjust the parameters (update rate, samples, ..) to avoid ghost objects:

![noise-laser](https://user-images.githubusercontent.com/1286618/108911725-ee5ff400-76