Improve facilities to run python engines locally

[thrau]

Some python engines have a script that allows running the engines locally (for example in facescv` and stream the plugged in camera directly. Currently the only reusable method for this is the stream_camera method, which has no way of interacting with the engine result, and which is a relic of early development. Also there is no way to use the automatic transformation pipeline.

It would be cool to adapt the srv.py facilities and the channels to implement a local "serving".