Complete rewriting of the code

Rewrote the code after @hunminkim98's tests on (1) angle visualization, (2) with RTMPose, (3) including on webcam.

Gave an impulse to implement many other changes:

• Used RTMPose rather than OpenPose or BlazePose (faster, more accurate, more flexible)
• Single process for both pose and angle estimation
• Webcam analysis available
• Body angle visualization
• Used entry points by default for clarity and simplicity
• Saved as OpenSim files: TRC for pose, MOT for angles
• Clearer documentation
• Better demo video
• Improved tracking IDs across frames
• Added some segment angles: Pelvis, Shoulders, Head