Update PoseTraining types

[rly]

Work in progress. This is a follow-up to #9 to get tests to pass and demonstrate plausible example usage code.

• Update existing tests
• Add unit tests for new types
• Update README with test_example_usage.py
• Touch base with ndx-pose users/developers to make sure the example code makes sense
• Discuss how storage of training frames in a multi-camera setup should work
• Update README with new contributors
• Update how devices are handled based on Support linking to multiple camera devices #5 and original_videos should be a link #12

This PR introduces some major changes from the hierarchical organization in #9 in order to organize similar objects into the same group.

[rly]

Will merge now so that @talmo @eberrigan can hack on this to update TrainingFrame to hold embedded images and multiple skeleton instances.