segment_videos function

[cellistigs]

I'm starting a pull request for the video segmentation we discussed. Nothing is implemented yet, but I've stubbed out the relevant functions in 'src/contrib/segment_videos.py'. Please feel free to add to this pr if you like.

[cellistigs]

I've added functions to segment the videos into n frames before running inference. I've also integrated this into the run_dgp_demo.py code, using two flags:

• --split is a boolean flag that tells the script to segment each video in the video_sets variable.
• --splitlength gives the length in frames of each video chunk.