support for multiple velodynes

Hello,

we have a scenario here with multiple velodynes in one network. So we developed a patch for your driver, which allows working with multiple velodyne LIDARs.

It adds the parameter "device_ip" to the velodyne_driver node. With the parameter one select which LIDAR's packets  should be processed and published by the node. Default behaviour is unchanged (process all packets).

Hope you find it useful.
Denis