FrSky LiPo sensor code refactored

Re #1429:
Added gtests for FrSKy LiPo sensor.
Refactored code, Cell and Cell- now calculated periodically
Corrrected indentation

I broke things in https://github.com/opentx/opentx/commit/36e78b93158ff7cbae99b32deefc3063c8f92195#commitcomment-6876849, therefore I wrote gtest for this part of the code and done another refactoring. According to the tests, it should work now. 

@kilrah please do a test with real sensor, I don't have one.