Bangle.js 2 Cutting Edge - new heart rate algorithm

thanks for the availability and the suggestions! You will be receiving pull requests soon 😃. I'll keep the signature as it is for now, and only change it if it is required.

One little note about the code, I see that you mostly use generic types like short, int, float etc. We will try to keep that style too, although I wonder if it wouldn't make more sense to be more specific and indicate also the n. of bits? What's the philosophy here?

Re. activity detection: it is not planned at the moment, but it's something I'd like to work on at some point.