Add Mux and Demux to hydro

I was missing a Demux tool. So I brought the old code back to life.
While working on that, I also fixed the service-servers of the Mux tool, so more than one Mux can be used.

Code is tested for hydro. But I think it can also be merged into newer versions. As I have no newer version running, I could not test. ==> Would be nice, if someone could do that.

If my request gets merged, I am happy to update the ROS WIKI page.