Added predefined distances

[javiber]

• added predefined functions
• allowed the Tracker to receive a distance by name
• renamed some of the distances to be more precise. For instance, what we were caling euclidean is actually frobenius
• small refactor to the tracker, separating the calculations of distances to a separate instance method. The motivation is that this will accommodate vectorized implementations easier by overwriting this new method.
• updated most of the demos, unifying some of the implementation implementations to ran Alphapose so I left it unchanged. motmetrics4norfair demo was also left unchanged because it uses a pretty custom distance, we can discuss if we want to incorporate it as another distance.