launch file and entrypoint to launch pyros server rosnode

[asmodehn]

My main current usecase is to launch pyros from rostful.

However launching via the python context is impractical in many other cases :

• from ROS, a user wants to roslaunch a node to start pyros
• from shell, a user wants to run a shell script (entrypoint) to start pyros
• from inside a python interpreter, launching only the server would also be useful.
  and these would be really helpful for debugging as well.

So :

• implement entry point to a script to launch pyros (maybe only in ROS version if we split ?)
• implement launch file to start server (via entrypoint script)
• move the contextmanager for pyros client to the only user at the moment (rostful), and only if pyros is not already started...

[asmodehn]

This probably require a way for the client node to :

• "discover" the pyros node in order to establish communication.
• connect to a well known (ZMQ) uri.
  Check Discovery between different process pyzmp#5 for more details...