Add start of support/testing with UP2500.

[abelsson]

I thought it might be useful to open a pull request for discussion rather than spamming other people's issues :)

I've added parsing which I think works for all of for US2000/US3000/UP2500 as well as a few simple tests to make sure the code behaves as expected to help future refactoring.

I don't have a US2000/US3000 (or any multi battery configuration) to test with, so it someone could verify it works on real devices it would be helpful!

Again, many thanks for this code! It's great that I managed to talk to my battery with relatively little effort thanks to this :)

[abelsson]

I think this might fix the issues with mixing US3000 and US2000 mentioned in #2 and the US3000 problems mentioned in #4.

[Frankkkkk]

Merged via #17; thanks !