tracker.py downstream compatibility + other fixes

[ahmed-alhindawi]

The main aim of this pull request is to maintain compatibility with downstream projects that rely on the tf frames and topics published. It also cleans a little bit of code to maintain readability/performance in some regards.

• changed isinstance_tracked_element as an if-statement into entry point of element rather than at every insertion/update inside a private function
• changed uuid to index to maintain backward compatibility
• insertion of new element now is checked rather than try-catch to improve readability and performance

Let me know what you both think @Tobias-Fischer @Twarz

-NgageOrange

[KevinCortacero]

Regarding #21, I agree with your implementation and fixes :)

Best, Kévin