Extract method _run_no_output for two calls to ffmpeg

Fix feature.path → feature.source.path after Feature API change

Handles serialisation of custom classes via to_json() method if present

Outputs a representation of the Features extrated by the pipeline

Intent is to write a FE that takes the output so that a pipeline can be
're-run'

Output JSON could also be used with external tools

Annotate features, label axes

Creates a plot showing where in source video(s) the pipeline features
came from

This pvaes the way for parts of the pipeline that do not produce videos,
such as JSON, images, clips etc

TODO: rename module video_producers → producers

Classes for actually making video files

 - VideoProducer -- interface
 - FfmpegVideoProducer -- make a temporary video using ffmpeg