Retrieval of camera geometry in event loop

In both process_dataset_mc and process_dataset_data of hillas_preprocessing.py, the following line throws an error:

    camera = event.inst.subarray.tel[tel_id].camera

Indeed, subarray in ctapipe is not anymore a property of each event, but of the particular event source (see https://github.com/cta-observatory/ctapipe/pull/1157).