* Make statistics state characteristic selectable
* Move computation in helper function
* Add relevant config elements for clarity
* Rename variables for better readability
* Avoid reserved prefix ATTR_ for stats
* Fix NoneType base_unit error
* Add testcases for statistics characteristic
* Add testcases for state_class, unitless, and characteristics
* Add testcase coverage for no unit with binary
* Replace error catching by an exception
* Attend to review comments