Move tracker dependencies to extras

[philipqueen]

Fixes #23.

Removes all of the pose estimators/tracking algorithms from the core dependencies and moves them into extras. Download options would be:

pip install skellytracker - just the core package
pip install 'skellytracker[mediapipe]' - core package and mediapipe
pip install 'skellytracker[yolo]' - core package and ultralytics (YOLO)
pip install 'skellytracker[all]' - core package, mediapipe, and ultralytics/YOLO

this is very easy to maintain and add to. Freemocap can just import [all] for now, but it gives us an option to change the download if ever include less commonly used trackers.

I've had to add conditional imports to files that download multiple trackers, but everything successfully runs. You can execute the RUN_ME and process_folder_of_videos in the limited install versions as long as you're using a tracker that's installed (e.g. missing YOLO doesn't throw an error when you try to run mediapipe).

[jonmatthis]

Put some newlines around the "need to install" print statements to make them easier to see among the terminal output

[philipqueen]

Put some newlines around the "need to install" print statements to make them easier to see among the terminal output

Done! Also Github added a cool new "batch suggestion" option where you can add all the suggestions to a batch and add them in one commit