fix retrieving a proxy

At least in case of my installation, get_proxy() as it has been used in camera_utils.py was not working.

In case get_proxy() is indeed no longer working, the merge request therefore suggests to use code similar to the one in the ArViz client. If the request gets accepted, this would also affect the documentation, in particular at https://armarx.humanoids.kit.edu/python/examples.html#retrieving-a-proxy.